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

Usage

# conf.py
linkcheck_ignore_case = True