Add indexes, update homepage links, various other fixes.

[kattni]

• Adds indexes for every top-level category, and links them from the main page.
• Updates two configuration settings; one in config.yml, one on the constants class call.
• Moves about/community.md content to main page, removes file.
• Removes reference to rST in contribution guide.

TODO:

• Please verify the page descriptions on reference/index.md, about/index.md and topics/index.md.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

TODO content has been reviewed; it all looks good. I've pushed an update to correct the development instructions; other than the linking approach which @HalfWhitt has already flagged, this looks good to me.

[HalfWhitt]

Thanks for catering to my/Russ's capricious demands. 😉 Just a couple of IDs / anchor cross references that are still kicking around.

[HalfWhitt]

Looks great to me!