docs: Revise and rework user-guide/expressions

[rodrigogiraoserrao]

PLEASE do not review this PR by looking at the diff.
Checkout the PR and build the docs with mkdocs serve.

TODO:

• revise page on expression plugins
• revise page on user-defined Python functions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.21%. Comparing base (8a76dad) to head (4041c8b).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19360      +/-   ##
==========================================
- Coverage   80.24%   80.21%   -0.04%     
==========================================
  Files        1523     1523              
  Lines      209545   209853     +308     
  Branches     2434     2434              
==========================================
+ Hits       168148   168324     +176     
- Misses      40842    40974     +132     
  Partials      555      555              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.