Support multiple MkDocs themes & styling from the same branch

[benkirk]

This PR supports building different styles from the same branch by doing some tree reorg and using environment variables

Functionally this means we don’t need multiple branches to support the primary and iframe deployments.

[benkirk]

The approach is as follows:

1. Reorganize the source tree:

   mkdocs.yml
   theme.yml -> themes/NCAR_primary.yml
   themes/ 
   ├── NCAR_primary/
   │   ├── assets/
   │   │   └── [...]
   │   ├── css/
   │   │   └── NCAR_primary.css
   │   ├── main.html
   │   └── partials/
   │       └── footer.html
   ├── NCAR_primary.yml
   ├── arc_iframe/
   │   ├── assets/
   │   │   └── [...]
   │   ├── css/
   │   │   └── arc_iframe.css
   │   ├── main.html
   │   └── partials/
   │       ├── footer.html
   │       ├── header.html
   │       ├── logo.html
   │       ├── nav-item.html
   │       ├── nav.html
   │       └── source.html
   └── arc_iframe.yml
   

   mkdocs.yml uses the INHERIT: option to include theme.yml, which is in turn a symbolic link into the themes/ directory specifying which theme should be built.

2. Use ReadTheDocs user-defined environment variables to distinguish the DEPLOY_TARGET at build time (primary site, iframe, etc...)

    # deploy the primary site: https://readthedocs.org/projects/ncar-hpc-docs/
    DEPLOY_TARGET=NCAR_primary
    
    # build the ARC iframed style site: https://readthedocs.org/projects/ncar-hpc-docs-arc-iframe/
    DEPLOY_TARGET=arc_iframe
   

3. Use a ReadTheDocs pre_build rule to run a make command to update the appropriate symbolic links based on the DEPLOY_TARGET:

   # DEPLOY_TARGET is set in our readthedocs admin interface, and should
   # be one of the supported targets: [ NCAR_primary, arc_iframe ]
   make ${DEPLOY_TARGET}-stylelinks
   

   Note: This make step could be avoided with proper support for the !ENV mkdocs environment variable support, however at the time of this writing using this inside mkdocs.yml causes build failures (example) on the readthedocs build images. This feels like a bug and one I'd like to resolve, but likley will require a small stand-alone test case and support from ReadtheDocs.

4. Right now the symbolic link theme.yml is committed to our repo, so a fresh clone will work with no environment variable nor make nonsense. This should not be changed, however, and we have a github action in place to be sure it points to the right location (theme.yml -> themes/NCAR_primary.yml). This complication could be overcome if/when we can use the !ENV yaml tag.

5. Testing this required a separate ReadtheDocs instance, see for example https://ncar-hpc-docs-arc-iframe--108.org.readthedocs.build/en/108/

---

Finally, another change was implemented in this whole process. Since our iframed site is a complete duplicate of content on the primary ReadTheDocs site, we don't want search engine indexing or caching. This can be achieved with the following meta tag, which should NOT be present on NCAR_primary but included in any other restyled builds:

    <meta name="robots" content="noindex,nofollow,noarchive">

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