Auto link extras: fail on bad path

[ftes]

Verify paths in links to :extras pages when the link includes a directory component.
Doesn't affect bare filename links such as test.md.

Before

Before this change, ExDoc effectively matched extra links by Path.basename(path), so a link like [Guide](guides/bad_dir/guide.md) could still resolve if there was any extra named guide.md in the flattened output namespace.

After

After this change, ExDoc keeps bare filename links as the legacy flattened lookup, but treats links with a directory component as path-qualified and resolves them through the extra source tree instead.

Design Decision

The main design question in this PR is how to interpret links to extras depending on the shape of the markdown path.

Link form	Example	Lookup base before	Lookup base after
Bare filename	[Intro](intro.md)	Flattened output filename	Flattened output filename
Implicit path	[Intro](guides/intro.md)	Flattened output filename	Current extra source file directory
Explicit relative path	[Intro](../guides/intro.md)	Flattened output filename	Current extra source file directory
Explicit absolute-style path	[Intro](/guides/intro.md)	Flattened output filename	Project root

More concretely, if the current extra source file is guides/current.md:

Link	Resolves after this PR
intro.md	legacy flattened lookup for intro.md
./intro.md	guides/intro.md
guides/intro.md	guides/guides/intro.md
../guides/intro.md	guides/intro.md
/guides/intro.md	guides/intro.md from project root

So guides/intro.md is not project-root-relative. Only /guides/intro.md is.

This keeps the legacy behavior for bare filename links while making path-qualified links fail loudly when the specified path is wrong.

[github-actions]

📦 Docs artifacts are ready: https://github.com/elixir-lang/ex_doc/actions/runs/23216300678/artifacts/5973917985