Add linkcheck_ignore_case configuration option

[FazeelUsmani]

This PR adds a new configuration option linkcheck_ignore_case to enable case-insensitive URL and anchor checking in the linkcheck builder.

Problem

Some web servers (e.g., GitHub, certain hosting platforms) are case-insensitive and may return URLs with different casing than the original link. This causes the linkcheck builder to report false-positive redirects when the URLs differ only in case, even though they point to the same resource.

Solution

• Added linkcheck_ignore_case boolean configuration option (default: False)
• Modified URL comparison logic to support case-insensitive matching when enabled
• Modified anchor comparison in AnchorCheckParser to support case-insensitive matching when enabled
• Added comprehensive tests for both URL and anchor case-insensitive checking
• Updated documentation in doc/usage/configuration.rst

[jayaddison]

Hi @FazeelUsmani - thank you for developing and describing this pull request.

I have a concern that enabling the option reduces the precision of other hyperlinks that are checked.

Could you explain the use case where it would be easier for a documentation project to enable this option by editing the conf.py instead of fixing the URLs/anchors in their documentation sources to use the correct casing?

[FazeelUsmani]

That’s a good point, @jayaddison.
This option is off by default and meant mainly only for large or older docs where many links hit servers that normalise URL casing (like GitHub) or are case-insensitive (like Windows). Enabling it just filters out harmless casing-related redirects so teams can focus on real link issues instead of noise.

[jayaddison]

@FazeelUsmani got it, understood. As often happens, I had a misunderstanding to begin with - you are saying that this only affects whether case-adjusted response URLs are considered to be redirect instead of successful.

Let me think about this a little more; I do understand the value in this now, but am wary of (and trying to think of) any problem side-effects.

[jayaddison]

(also, thank you for the explanation)

[jayaddison]

Separately: I do think that we should probably isolate the redirect-case-sensitivity handling from the HTML anchor case-sensitivity; they seem fairly functionally different from each other to me.

[FazeelUsmani]

Hmm.. makes sense. I can refactor this into two separate options:
linkcheck_ignore_case_urls: For comparing URL paths (the redirect scenario)
linkcheck_ignore_case_anchors: For comparing HTML anchors

This would give users more granular control. Most users would likely want linkcheck_ignore_case_urls = True (for case-insensitive servers) while keeping linkcheck_ignore_case_anchors = False (since HTML IDs are technically case-sensitive per spec). What do you say?

[AA-Turner]

Two options seems overkill for this use-case. What do browsers do de facto on case mismatches on fragment IDs?

A