docs: Some clean up to Jekyll site

[crandmck]

This PR contains some improvements to the Jekyll site generation that we can leverage when we fully automate the API doc generation and ultimately remove the TypeDoc-generated .md files from the repo. This is a step in that direction.

• Changes the build:docs command in package.json so that building docs won't delete or overwrite the files used for the Jekyll/GHP site. It should only generate/update .md files in docs/classes, docs/functions, docs/interfaces, and docs/type-aliases.
• Replaces manually-written list of pages with programmatic list of links to the files in docs/classes, docs/functions, docs/interfaces, and docs/type-aliases
• Adds an docs/index.md which is the API doc landing page. README.md is used for the sidebar content.
• Ensures that the nav items displayed in the top right contain only the links to Classes, Functions, etc.