Check the built docs for broken internal links

[pvcraven]

Closes #1168. Part 2, after #2974 fixed the existing broken links.

uv run make.py docs-full, which CI's docs job runs, now runs util/check_internal_links.py build after the Sphinx build. It fails the build if any page links to a page, file or anchor that doesn't exist.

Why

Since #2970, nitpicky mode catches broken cross-references (:ref:, :py:class: and so on). It doesn't see plain links, which is how these got through:

• the 12 broken gallery thumbnail links fixed in Put example and tutorial pages in the docs navigation #2968
• 19 named links missing their trailing underscore, such as `Discord <Arcade Discord>`_, fixed in Fix broken links in the docs #2974. docutils silently treats the name as a relative URL.

The script checks the HTML Sphinx actually wrote, so it catches any of these, whatever the source.

How it works

• It scans every built page except Sphinx's generated _modules/, _static/, _sources/, genindex and search pages.
• For each href/src, it checks that the target file exists. For #anchor links to an HTML page, it checks that the page has that id.
• It skips external links (http, https, mailto, data) and cache-busting query strings (?v=...).
• Every page repeats the sidebar, so it checks each link once per folder. A full build (859 pages) takes about 2 seconds.
• It uses os.path.normpath, not Path.resolve(). resolve() hung on some links on Windows.
• It also adds the missing import sys to make.py, which the new line needs.

Per #1168's discussion, this doesn't check external links. Those go down unpredictably, so make.py linkcheck stays a manual command.

Tested

• uv run make.py docs-full from a clean build/ passes. The Sphinx build plus the check report "No broken internal links".
• On a build from before Fix broken links in the docs #2974, the script reports the 20 broken links Fix broken links in the docs #2974 fixed and exits 1. run() in make.py exits with a failing command's status, so docs-full fails too.
• ruff format and lint pass.

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