Enhance documentation

[munrojm]

This PR greatly enhances the documentation for the API seen on the auto-generated Swagger and Redoc documentation pages:

• Overall description
• Enhanced tag descriptions with outside doc links
• Enhanced GetByKey descriptions with fields

[codecov-commenter]

Codecov Report

Merging #354 (8e86347) into main (8db87c4) will decrease coverage by 0.04%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##             main     #354      +/-   ##
==========================================
- Coverage   93.17%   93.12%   -0.05%     
==========================================
  Files          60       61       +1     
  Lines        1950     1965      +15     
==========================================
+ Hits         1817     1830      +13     
- Misses        133      135       +2     

Impacted Files	Coverage Δ
src/mp_api/core/documentation.py	0.00% <0.00%> (ø)
src/mp_api/core/api.py	100.00% <100.00%> (ø)
src/mp_api/core/settings.py	100.00% <100.00%> (ø)
src/mp_api/matproj.py	98.52% <0.00%> (+0.14%)	⬆️

---

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 8db87c4...8e86347. Read the comment docs.