fix(rtd) Restore the requirements.rtd.txt file

[Manangka]

In a previous commit the requirements.rtd.txt had been removed in favor of creating a pixi rtd environment. However the ReadTheDocs site doesn't support pixi and requires either pip or conda.

This commit restores the requirements.rtd.txt to satisfy that requirement

Checklist of items for pull request

• Closed issue #xxxx
• Referenced issue or pull request #xxxx
• Added new test or modified an existing test
• Ran black on new and modified autotests
• Formatted new and modified Fortran source files with fprettify
• Added doxygen comments to new and modified procedures
• Updated meson files, makefiles, and Visual Studio project files for new source files
• Updated definition files
• Updated develop.tex with a plain-language description of the bug fix, change, feature; required for changes that may affect users
• Updated input and output guide
• Removed checklist items not relevant to this pull request

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