Overhaul documentation

- [x] Migrate docs to new Sphinx
- [x] Fix docs deployment to website
- Ideally this includes a way to deploy a beta version of our docs
- [x] Switch docs from automodule to autodoc2
- [x] Deploy skeleton layout
- [x] Migrate existing docs to new layout
- [ ] Write new docs where we have gaps
- Megatron in 15 minutes
- ProcessGroupCollection
- ...
- [ ] Refresh docs
- [ ] `docs/api-guide/models/models.md`
- [ ] `megatron/core/transformer/moe/README.md`
- too long but has good content
- [x] Make MLM README succinct
- [x] Publish quarterly roadmap
- [ ] Publish first blog post about a new feature
- [ ] Track doc metrics
- e.g. page views
- [ ] https://github.com/NVIDIA/Megatron-LM/issues/2653

On the fence:

- [ ] Force docs to be within `docs` top-level directory
- [ ] Add @megnvidia to all PRs with `docs-only` label