Update CONTRIBUTING and gitignore for docs build (#47539)
Summary:
This PR tries to make building the docs less confusing for new contributors:
- `npm` is discouraged on devservers for Facebook employees, so I added another way to install `katex`
- the path to `check-doxygen.sh` was wrong, so I fixed it
- while generating the CPP docs, it created two new folders that weren't ignored by Git, so I added those to `.gitignore`
- I wasn't able to get the SSH tunnel to work, so I added instructions to use `scp` as an alternative
I'm not entirely sure how the `docs/cpp/source/{html,latex}/` directories were created since I haven't been able to reproduce them.
I also think that it would be better to use the SSH tunnel since `scp` is so much slower, but I just wasn't able to figure it out; I followed the instructions from `CONTRIBUTING.md` and then ran a [Python `http.server`](https://docs.python.org/3/library/http.server.html) on my devserver:
```bash
python -m http.server 8000 --bind 127.0.0.1 --directory build/html
```
but my browser failed to connect and my (local) terminal printed error messages (presumably from the SSH command).
If anyone knows how to properly set up the SSH tunnel and HTTP server, I add those more detailed instructions to `CONTRIBUTING.md` and remove the `scp` instructions from this PR.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47539
Reviewed By: malfet
Differential Revision: D24806833
Pulled By: samestep
fbshipit-source-id: 456691018a76efadde28fa5eb783b0895582e72d
