Smarter file retrieval

[abstractalgo]

• [x] generate rosetta folder structure as JSON at build time and use that, instead of querying the file system on demand
• [x] pull markdown files directly from Github instead of local file system (should be faster + caching/CDN)

[abstractalgo]

Done as part of https://github.com/abstractalgo/rosetta/commit/e3ec09fd65edc4605d635cf317f1655cba6afa6b

[abstractalgo]

First item done as part of https://github.com/abstractalgo/rosetta/commit/6d1b13255d2e4d5f264ff5c4cd1d6278e29f53e5