Fix Dependabot not creating new dependency PRs

[ddabble]

Description

Error message from Dependabot:

Dependabot couldn't fetch all your path-based dependencies
The affected dependencies were "-e ." at /requirements/docs.txt.

To use path-based dependencies with Dependabot the paths must be relative, resolve to a directory in this project's source code, and contain a valid Python project.

(Prompted by visiting Dependency graph -> Dependabot -> "Recent update jobs" next to requirements/coverage.txt -> "Check for updates".)

The fix simply moves installing the project locally from requirements/docs.txt to .readthedocs.yaml (not in editable mode (-e) anymore, but the build still succeeds - see below).

Related Issue

None

Motivation and Context

No new Dependabot PRs for our pip packages have been created for many months, leaving multiple packages outdated.

Context for the removed lines: bd26f19

How Has This Been Tested?

Evidence that RTD still builds successfully (for the commit ddabble@aa5e062)

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have run the pre-commit run command to format and lint.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have added my name and/or github handle to AUTHORS.rst
• I have added my change to CHANGES.rst
• All new and existing tests passed.