docs: add updatelinksonrename example plugin

[skirmess]

This adds an example plugin under docs/plugin_examples/ that updates WikiLinks when a page is renamed, implementing the request from #210 / #228.

What it does

On rename, the plugin hooks into page_renamed and rewrites every WikiLink ([[...]]) across the wiki that points to the old page so it points to the new name. Matching uses get_filename() (the same resolver Otter Wiki uses), so it is case-, slash- and .md-insensitive, and #anchors and explicit [[title|link]] titles are preserved.

Scope / limitations

• Only WikiLinks are rewritten — matching what the bundled referencingpages example can discover. Plain Markdown links ([text](/Page)) are intentionally left untouched.
• Exact match of the renamed page only; child pages and attachment links (Renaming attachments should updated links in pages. #65) are not touched (child pages aren't moved by a rename, so links to them stay valid).
• A per-rename checkbox (as floated in When a page is renamed, update links to it in other pages' files? #210) isn't exposable via the current plugin API, so an UPDATE_LINKS_ON_RENAME config flag (default on) lets you disable it without uninstalling — handy when the plugin is baked into a Docker image.

Tests

Adds tests/test_plugin_updatelinksonrename.py covering the rewrite (anchor + title preserved, unrelated links untouched) and the opt-out flag. I noticed the example plugins currently have no tests, so happy to drop it or move things around if you'd rather keep them test-free.

[redimp]

Hey @skirmess, thanks for this PR. Will test, review, and come back to you.

[ribbal]

My thoughts on this is that it should be part of the Otter Wiki core functionality. What do you think @redimp, @skirmess?

Or, am I fundamentally missing something?

I'm sure @skirmess's good work could still be re-used.

[skirmess]

Yes, that would probably make sense to have it in core (it's more a bug fix than a new feature). But I'm not a Python dev, I can't even really read it - and I'm not going to use AI to edit someone else's software if I don't understand what the change does. :-)

That's why I asked Claude to write a plugin. Because I wanted that feature now.

You can implement it in core, with or without using part of this MRs code. Or add the plugin to the repo. Or close the MR. All fine with me. Claude wrote it, I just tested it on my installation.