Match RTD directory URLs to TOC index.html

[aidanfnv]

The ReadTheDocs site is supposed to highlight the current page in the table of contents, but this does not work if the URL is a directory. ReadTheDocs will automatically resolve the page to the index.html file in the directory, but the script for highlighting the page in the TOC will fail to find the page because the directory URL does not match the URL in the TOC, which ends in index.html.

Example, the page ...external/core-module-reference/ does not match ...external/core-module-reference/index.html in the TOC.

This updates the matching logic to compare the URLs again after appending index.html, if there is not already a match.

See an example of this working at https://shader-slanggithubio-aidanfnv-wip.readthedocs.io/en/fix-directory-url-toc-highlight/external/core-module-reference/