Separate trees for user and developer docs

[ekapadi]

Short description of the changes:

This PR includes the following changes:

• Create separate trees for user-docs and developer-docs;

• Update .readthedocs.yml for the new locations;

• Fix pixi build-documentation tasks so that they actually work. In this regard please note that, for historical purposes, I've saved the original docs/rtd/Makefile to the new location docs/user/Makefile but I have not otherwise updated it.

Check list for the pull request

• I have read the [CONTRIBUTING]
• I have read the [CODE_OF_CONDUCT]
• I have added tests for my changes
• I have updated the documentation accordingly

Check list for the reviewer

• I have read the [CONTRIBUTING]
• I have verified the proposed changes
• best software practices
  • all internal functions have an underbar, as is python standard
  • clearly named variables (better to be verbose in variable names)
  • code comments explaining the intent of code blocks
• All the tests are passing
• The documentation is up to date
• code comments added when explaining intent

Manual test for the reviewer

It's important to verify that the pixi build-documentation commands actually work, these are:

build-dev-docs         Build the Developer documentation
build-docs             Build the User documentation
clean-docs             Clean the documentation build artifacts

References

EWM#12482

[walshmm]

This needs to be rebased against next.

I attempted to run pixi run build-docs after re-basing but received the following build error

✨ Pixi task (build-docs in default): sphinx-build -b html docs/user/source docs/_build/user: (Build the User documentation)
Traceback (most recent call last):
  File "/home/wqp/git/PyRS/.pixi/envs/default/bin/sphinx-build", line 6, in <module>
    from sphinx.cmd.build import main
  File "/home/wqp/git/PyRS/.pixi/envs/default/lib/python3.12/site-packages/sphinx/cmd/build.py", line 17, in <module>
    from sphinx.application import Sphinx
  File "/home/wqp/git/PyRS/.pixi/envs/default/lib/python3.12/site-packages/sphinx/application.py", line 21, in <module>
    from sphinx.environment import BuildEnvironment
  File "/home/wqp/git/PyRS/.pixi/envs/default/lib/python3.12/site-packages/sphinx/environment/__init__.py", line 23, in <module>
    from sphinx.transforms import SphinxTransformer
  File "/home/wqp/git/PyRS/.pixi/envs/default/lib/python3.12/site-packages/sphinx/transforms/__init__.py", line 21, in <module>
    from sphinx.util.i18n import format_date
  File "/home/wqp/git/PyRS/.pixi/envs/default/lib/python3.12/site-packages/sphinx/util/i18n.py", line 11, in <module>
    import babel.dates
ModuleNotFoundError: No module named 'babel'

[walshmm]

from a pixi standpoint this looks fine, through this will need a rebase to coalesce my dev packaging changes

[walshmm]

The tasks work and the docs build.

Though I thought this repo was configured to have a readthedocs job run on each pr...I dont have perms to change that so I guess we'll see if they build?
According to readthedocs this repo is associated with this link despite the name: https://app.readthedocs.org/projects/pyrs-testing/