Remove index.md files from being searched

[kaixin-hc]

What is the purpose of this pull request?
Fixes #1803

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• Others, please explain:

Overview of changes:
Adds the index files in devGuide and userGuide to the list of files which should not be indexed for search.

Anything you'd like to highlight / discuss:

Testing instructions:
Not sure how to test the ug-site and dg-site.json as opposed to site.json. I'm wondering if this is an area in the documentation we should improve.

Proposed commit message: (wrap lines at 72 characters)
Docs: Remove index.md files from being searched

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Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

[jonahtanjz]

Thanks @kaixin-hc! This seems to remove index.md from the search for both ug and dg.

Agree with #1803 (comment) that we should find a way to format the titles of the files as well if needed. Can be a separate PR to work on this :)

Not sure how to test the ug-site and dg-site.json as opposed to site.json. I'm wondering if this is an area in the documentation we should improve.

To test the ug-site and dg-site individually, you can try:

1. cd docs
2. markbind serve -s ug-site.json or markbind serve -s dg-site.json

Using the -s option allows you to choose which config file to use for serving/building the site. This is documented under the serve command but I guess we can include a section within the dev guide to show how each of the individual site can be served :)

LGTM 👍