Update docs workflows to publish to local repo

[kjmichel]

Docs are currently pushed to two locations:

fairchem/.github/workflows/deploy_docs.yml

Lines 26 to 38 in afba9d8

	- name: Deploy to ghpages branch
	uses: peaceiris/actions-gh-pages@v4
	with:
	github_token: ${{ secrets.GITHUB_TOKEN }}
	publish_dir: docs-html
	- name: Deploy to fair-chem.github.io
	uses: peaceiris/actions-gh-pages@v4
	with:
	deploy_key: ${{ secrets.ACTIONS_DEPLOY_KEY }}
	external_repository: FAIR-Chem/fair-chem.github.io
	publish_branch: gh-pages
	publish_dir: docs-html

1. A gh-pages branch in this repo, which exposes them at https://facebookresearch.github.io/fairchem/ (notice that this is broken though!)
2. The gh-pages branch of https://github.com/FAIR-Chem/fair-chem.github.io, which exposes them at https://fair-chem.github.io

This PR removes step 2 - updates will no longer go to fair-chem.github.io. It also fixes the broken site at https://facebookresearch.github.io/fairchem/ by setting the BASE_URL environment variable to this repo's name as expected by MyST (https://mystmd.org/guide/deployment#deploy-base-url).