Do not append `index` with `dirhtml`

[hugovk]

Steps to reproduce

1. Build docs using dirhtml that has ogp_site_url = "https://devguide.python.org/" in conf.py

   git clone https://github.com/python/devguide
   cd devguide
   make dirhtml
   # or:
   # sphinx-build -b dirhtml -d _build/doctrees  -W --keep-going -n . _build/dirhtml

2. Open a non-index page, for example _build/dirhtml/getting-started/getting-help/index.html and check og:url:

<meta property="og:url" content="https://devguide.python.org/getting-started/getting-help/" />

✅

3. Open a index page, _build/dirhtml/index.html and check og:url

Actual result

❌ Appends index to give a 404 URL: https://devguide.python.org/index/

<meta property="og:url" content="https://devguide.python.org/index/" />

Expected result

Don't append index for the main page:

<meta property="og:url" content="https://devguide.python.org/index/" />

See also

This came up at https://github.com/python/devguide/pull/953#issuecomment-1274377631, see the demo build at https://cpython-devguide--953.org.readthedocs.build/

[hugovk]

Also need to deal with other index pages, such as _build/dirhtml/getting-started/index.html:

<meta property="og:url" content="https://devguide.python.org/getting-started/index/" />

❌ https://devguide.python.org/getting-started/index/

✅ https://devguide.python.org/getting-started/

[hugovk]

PR https://github.com/wpilibsuite/sphinxext-opengraph/pull/78 updated.