[ci] update some CI dependencies, add dependabot

[jameslamb]

• adds configuration to get dependabot to automatically recommend updating third-party GitHub Actions
• changes readthedocs configuration to "latest" types of versions that'll keep automatically updating as readthedocs does
• updates pre-commit versions (ran pre-commit autoupdate)
• switches commands from mamba {cmd} back to conda {cmd}
  • conda now includes libmamba as its default solver, no more need to use the mamba command

References

• https://docs.readthedocs.io/en/latest/config-file/v2.html#build-tools-python
• https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
• https://conda.org/blog/2023-11-06-conda-23-10-0-release/