Fool-proof submodules local/readthedocs.org integration

[michaelweinold]

@tngTUDOR mentioned the following potentially confusing case in #31 (comment):

1. a contributor makes a change to a function docstring in eg. bw-calc, commiting his change to the main branch
2. to check this change in the documentation Sphinx build, he pulls thesese changes into the bw-calc submodule in the brightway-documentation repo with git pull --recurse-submodules
3. building the documentation locally, the changes are all present. however, unless the updated commit ID in the bw-calc submodule are commited+pushed, readthedocs.org will not have the changes until dependabot updates the submodules on remote.

This means that local/readthedocs.org versiond might be out-of-sync for a short time (until dependabot resolves these dependencies). This is true only for documentation created automatically from docstrings using AutoAPI.

[michaelweinold]

Mitigation strategies:

1. update the "documentation documentation" to include this case and give clear instructions on how to avoid this scenario
2. auto-update the submodules on remote whenever changes in their respective upstream repositories are detected

Documentation:

1. N/A
2. Automatically updating git submodules using GitHub Actions or this SO thread (with more information on fine-grained access for machine users in this SO thread).

[michaelweinold]

Compare my own question: Trigger workflow in other repo using workflow_dispatch and GITHUB_TOKEN and my comment in a related discussion.

Compare recent GitHub functionality updates: GitHub Actions: Use the GITHUB_TOKEN with workflow_dispatch and repository_dispatch

[michaelweinold]

Issue: GITHUB_TOKEN is valid only within the repository of the workflow.

[michaelweinold]

Add GH action to the following repos:

• bw2calc
• bw2io
• bw2analyzer
• bw2data

[michaelweinold]

Check:

• GitHub Actions: Use the GITHUB_TOKEN with workflow_dispatch and repository_dispatch works only within a repository, not for triggering a workflow in another repository