[release/2.0] Move documentation from readthedocs to Wiki

[climbfuji]

Description

This PR removes the entire contents of doc and the associated entry in .gitmodules for release/2.0. This PR will later be merged back to develop, together with other PRs for the release branch.

The new documentation will be entirely in the spack-stack Wiki. Until the PR is approved and merged, the updated documentation can be found at https://github.com/climbfuji/spack-stack/wiki.

Dependencies

To merge this PR:

• Review the new documentation currently available at https://github.com/climbfuji/spack-stack/wiki and approve this PR once satisfied
• Fix any issues with GitHub actions integration of automated spack-stack readthedocs build
• Update the readthedocs config to only provide documentation for previous release (i.e. remove any develop documentation).
• Force-push the documentation from https://github.com/climbfuji/spack-stack/wiki to https://github.com/jcsda/spack-stack/wiki
• Incorporate updates from Configuration updates for discover-scu17 #1854 in the new spack-stack Wiki documentation, close Configuration updates for discover-scu17 #1854 as unmerged
• Close Update docs NewSiteConfigs.rst for spack v1 #1823 and Documentation update for Chapter 5. Adding Test Packages (Chained Environment)  #1853 as unmerged - both updates are already incorporated in the new spack-stack Wiki
• Merge this PR
• Update the release tasklist issue accordingly (WIP - Update release_tasklist.md issue template #1836)
• Update pull request template accordingly (no PR yet)

Issues addressed

Closes #1749
Closes #1761
Working toward #1835

Applications affected

None directly

Systems affected

None directly

Testing

• CI: Note whether the automatic tests (GitHub actions tests that run automatically for every commit) pass or not
  • GitHub actions CI tests pass
  • GitHub actions CI tests do not pass (provide explanation)
  • GitHub actions CI tests skipped (provide explanation if necessary)
• New tests added: List and describe any new tests added to GitHub actions
  • ...
• Additional testing: Add information on any additional tests conducted
  • ...

Checklist

• This PR addresses one issue/problem/enhancement or has a very good reason for not doing so.
• These changes have been tested on the affected systems and applications.
• All dependency PRs/issues have been resolved and this PR can be merged.
• All necessary updates to the documentation on readthedocs are included in this PR
  • For site config updates, check in particular doc/source/PreConfiguredSites.rst and doc/source/MaintainersSection.rst
• All necessary updates to the spack-stack wiki will be made when this PR is merged