Write utility documentation

[jlaehne]

Description of the change

Filled the different proposed sections of utilities.rst

Progress of the PR

• [x] Change implemented (can be split into several points),
• [x] Check whether it is displayed correctly,
• [x] added line to CHANGELOG.md,
• [x] ready for review.

[codecov-commenter]

Codecov Report

Merging #128 (b88e17a) into main (29312ea) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #128   +/-   ##
=======================================
  Coverage   96.69%   96.69%           
=======================================
  Files          11       11           
  Lines         575      575           
=======================================
  Hits          556      556           
  Misses         19       19           

Impacted Files	Coverage Δ
lumispy/signals/common_luminescence.py	96.96% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 29312ea...b88e17a. Read the comment docs.

[jlaehne]

@jordiferrero would be nice if you could provide a brief documentation on solve_grating_equation as a follow up.

[jlaehne]

The build documentation can be found here: https://lumispy--128.org.readthedocs.build/en/128/user_guide/utilities.html

No idea why the last run failed on the hyperspy dependencies, but changes were minor and should not affect the way it appears notably.

[jordiferrero]

@jordiferrero would be nice if you could provide a brief documentation on solve_grating_equation as a follow up.

Yes I should. It is in my todo list. Basically it follows the Horiba equiations. If you accept this one, then I can open a new PR soon with a more detailed explanation (need to get @Gunnar help). I guess for now check my comment.