Simplify contributing

[fyliu]

Fixes #267

What changes did you make?

• Moved CONTRIBUTING.md sections into files and created links from the original file
  • The original file became the section index page.
• Created contributing/ directory and moved contributing files inside
• Moved tools/ and howto/ (Developer How-to Guides) into contributing/
• Updated links for the above changes.

Why did you make the changes (we will use this info to test)?

• To split the long contributing docs file into topic pages so it'll be easier to add and make changes going forward.
• To organize contributing pages into the Contributing section. Leave main sections for user/customer documentation.

Screenshots of Proposed Changes Of The Website

Before link

Visuals before changes are applied

• Boxes show where the pages were originally.
• The page table of contents used to be on the right.
• There were numbering in the titles, because the document was so long
  • Having numbers as part of the titles (rounded boxes) made it difficult to insert new content because of having to re-number everything after it.

After link

Visuals after changes are applied

• Now, all contributor pages are organized under the "Contributing" section (boxed), leaving the main sections for "customers" (API client developers).
• There's a Contributing index page with links pointing to the other pages.

• Now, page table of contents are on the left, which frees up the right-hand space for the main content.