Small improvements to docs

[hakonanes]

Description of the change

Fixed a link to kikuchipy's contributing guide in the LumiSpy contributing guide by making it "dynamic" with Intersphinx. Also fixed the Sphinx version in setup.py so installing in editable mode with pip install -e . from root works.

Progress of the PR

• [x] Change implemented (can be split into several points),
• [x] update user guide
• [ ] added line to CHANGELOG.md,
• [x] ready for review.

[hakonanes]

For completeness, this was the initial error I got when trying to install a development version of LumiSpy in editable mode:

> pip install -e .
Obtaining file:///home/hakon/kode/lumispy
  Preparing metadata (setup.py) ... error
  error: subprocess-exited-with-error

  × python setup.py egg_info did not run successfully.
  │ exit code: 1
  ╰─> [3 lines of output]
      /home/hakon/miniconda3/envs/ls-dev/lib/python3.10/site-packages/setuptools/config/setupcfg.py:520: SetuptoolsDeprecationWarning: The license_file parameter is deprecated, use license_files instead.
        warnings.warn(msg, warning_class)
      error in lumispy setup command: 'extras_require' must be a dictionary whose values are strings or lists of strings containing valid project/version requirement specifiers.
      [end of output]

  note: This error originates from a subprocess, and is likely not a problem with pip.
error: metadata-generation-failed

× Encountered error while generating package metadata.
╰─> See above for output.

note: This is an issue with the package mentioned above, not pip.
hint: See above for details.

Both the warning (license_file -> license_files) and the error (remove extra ">=") are fixed in this PR.

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (b402c96) 100.00% compared to head (8bf558b) 100.00%.

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Additional details and impacted files

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