Add absolute links via output.html.site-url

[JesusPerez]

Closes #1764.
Supersedes #1802 (a reimplementation of @joaofreires' approach on the current crates/ workspace layout, with the review feedback addressed).

Problem

When a book is hosted in a subdirectory (or any page is reached at a deeply nested path), mdBook's depth-relative links can resolve incorrectly. output.html.site-url already exists but only feeds the <base href> of the 404 page — ordinary navigation, sidebar, asset and in-content links are not affected. See #1764.

What this does

When output.html.site-url is set (e.g. https://example.com/docs/), every generated link is rooted at that URL so the book resolves correctly regardless of the page's own depth:

• In-content links — root-relative ./path links in chapter bodies (and images / raw HTML) are anchored to site-url. Scheme links (https:, mailto:) and fragments are left untouched.
• Page chrome + JS sidebar — path_to_root is set to site-url, so prev/next, the print link, search and the toc.js-injected sidebar all become absolute.
• No-JS sidebar fallback — toc.html (the iframe fallback) gets a <base href> of site-url; this is scoped to that render so it never leaks onto regular pages (which would break their page-relative content links).
• Static assets — the {{resource}} helper honors the page's path_to_root, so hashed CSS/JS are emitted absolutely too.
• Print page — print.html chrome/assets are rooted at site-url, and cross-chapter references between chapters present on the page are still folded into intra-page #id anchors, keeping the consolidated page self-contained.

During mdbook serve the book is hosted at the local server root, so site-url is overridden to / (host-agnostic, resolves locally). This was previously silent; it now logs once, and a new --preserve-site-url flag keeps the configured value to preview the production absolute links locally.

Addressing the #1802 review

ehuss's review point on #1802	Status here
"This will need some tests to be added."	7 integration tests + 4 unit tests (see below).
"links on the print page aren't using the adjusted links."	print.rs::rewrite_links is now site-url-aware; cross-refs fold to anchors, externals stay absolute.
"there are some merge conflicts."	N/A — clean reimplementation on current master.
"HtmlConfig is public API → a new field is a semver break; use a wrapper."	No longer applies: in the crates/ layout site_url is already a field on HtmlConfig. No public API fields were added; threading is via internal Option<&str> parameters. cargo semver-checks passes.
"use if let over match, Option<&str> over Option<&String>."	Uses Option<&str> (as_deref()) and let-chains throughout.
"the new option needs to be in the TOML config docs."	Updated (see Docs).

Implementation

All link handling lives in mdbook-html; no public API was added.

• crates/mdbook-html/src/html/tree.rs — fix_link/fix_html_link take site_url; root-relative ./ links are anchored. Threaded at the three call sites via self.options.config.site_url.as_deref().
• crates/mdbook-html/src/html_handlebars/hbs_renderer.rs — path_to_root = site_url per chapter (and the index.html variant); base_url = site_url scoped to the toc.html render; print page rooted at site_url.
• crates/mdbook-html/src/html_handlebars/helpers/resources.rs — {{resource}} honors @root/path_to_root with a fallback (byte-identical output when site-url is unset).
• crates/mdbook-html/src/html/print.rs — render_print_page/rewrite_links take site_url; the site_url prefix is stripped and the remainder resolved as root-relative so in-print cross-refs still fold to #id.
• src/cmd/serve.rs — --preserve-site-url flag; the / override is now logged and opt-out.

Tests

• tests/testsuite/rendering.rs — site_url_* (7): content link rewriting, external links untouched, absolute path_to_root, absolute assets, <base href> only on toc.html, no <base href> leak onto chapter pages, opt-in behavior when unset, and the print page (chrome absolute + cross-refs folded to anchors).
• crates/mdbook-html/src/html/tree.rs — fix_link_tests (4): .md→.html, schemes/fragments left alone, ./→site-url, bare-relative links unchanged.
• Fixture: tests/testsuite/rendering/site_url/.

cargo test --workspace, cargo clippy --all-targets and cargo fmt --check all pass.

Docs

• guide/src/format/configuration/renderers.md — the site-url bullet now describes whole-site absolute links and the serve override.
• guide/src/cli/serve.md — --preserve-site-url section.

Notes

• The root-relative convention is the explicit ./path form, matching the original patch's semantics; bare relative links keep their page-relative meaning.
• A ./ link to the first chapter (which mdBook aliases README → index) stays an absolute link to its standalone page in the print output rather than folding to an anchor. This is a pre-existing README/index aliasing quirk, independent of this feature.
• Relationship to canonical-site-url (Add canonical-site-url setting #2706): the two settings are complementary and do not overlap. site-url is the deployment location and, with this change, the root for every emitted link/asset so the book resolves under a sub-path. canonical-site-url only emits <link rel="canonical"> for SEO and does not affect link resolution. They touch different code paths and can land independently.
• CHANGELOG.md intentionally left untouched (maintained at release time).

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Supersedes #1802; builds on @joaofreires' original approach there.