Polish docs; Automatically publish docs in CI

[humpydonkey]

Changes

1. Automatically publish docs in CI;
2. Polish docs;

Notes:

1. Add or rename a module requires updating the docs under docs/api.
2. docs/index.md needs to be in-sync with README.md (as it is copied from it). Need to find a better way to avoid copying it.
3. Renamed the old doc, i.e. docs/index.md to docs/old.md`. We can later re-purpose it to a user guide.