docs: Fixes to hierarchy and indentation

[scmcca]

• Fix list indentation in "Community" section so different parsers can pick up the nested indentation. The extra indentation spaces I proposed are consistent with other areas of the documentation (i.e., "Other multimodal data formats" section).
• Make programming language titles as a sub-heading of "GTFS Libraries". Currently, the programming language titles are treated as the same level. An alternative approach would be to bold the programming language titles. No strong opinion on either approach.
• Fix double bullet in "Other multimodal data formats" section.

[barbeau]

Thanks @scmcca! I fixed another indentation error in https://github.com/CUTR-at-USF/awesome-transit/pull/205/commits/62a25d895db13b35d4b499eb58fc1015e1ebf1fa, and am going to merge all this now. Please let me know if that looks ok.