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