Sync dev to main: link embed metadata - #1091

Three gaps left over from the social card work, all visible whenever
someone posts a wiki link in Discord.

97 pages carried no description at all. They have no `description` in
their front matter, and `site_description` was never set, so the fallback
resolved to nothing and the embed printed a bare title. That was
survivable when the card was a full-width banner with its own text. Now
that the image is a small generic mark, the description carries the page,
so add a site-wide sentence for those pages to fall back to.

Add `og:site_name`, so the small provider line above the title reads
"Apollo Docs" instead of the raw wiki.apolloautomation.com. With that in
place the " - Apollo Docs" suffix on every title repeats the line directly
above it, so drop the suffix and give the title the full width before it
truncates. The homepage still resolves to "Apollo Docs" once.

Add a `theme-color` tag, which Discord uses to tint the stripe down the
left edge of an embed and which Android uses to tint browser chrome. The
stripe is drawn on every embed regardless; without this tag it is
Discord's default grey. Material emits no such tag here because the
palette uses a custom primary rather than a named color. It has to be
`name=` rather than `property=`, so it cannot come from the card layout,
which hardcodes `property=` for everything. It goes in overrides/main.html
instead. Material's stock main.html is a bare `{% extends "base.html" %}`,
so overriding it drops nothing.

Verified by building all three page shapes and reading the emitted tags: a
page with its own description keeps it, a page without one picks up the
new fallback, and the homepage title does not double or empty out.

🤖 Generated with [Claude Code](https://claude.com/claude-code)