Add a new Exceptions section in documentation and document UsageError

[Pulkit07]

This PR fixes #4560

• Create a new changelog file in the changelog folder, with a name like <ISSUE NUMBER>.<TYPE>.rst. See changelog/README.rst for details.
• Target the master branch for bug fixes, documentation updates and trivial changes.
• Target the features branch for new features and removals/deprecations.
• Include documentation when adding new features.
• Include new tests or update existing tests when applicable.

Unless your change is trivial or a small documentation fix (e.g., a typo or reword of a small section) please:

• Add yourself to AUTHORS in alphabetical order;

[codecov]

Codecov Report

Merging #5231 into master will decrease coverage by 1.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5231      +/-   ##
==========================================
- Coverage   93.33%   92.26%   -1.08%     
==========================================
  Files         114      115       +1     
  Lines       25493    26159     +666     
  Branches     2480     2578      +98     
==========================================
+ Hits        23795    24135     +340     
- Misses       1379     1702     +323     
- Partials      319      322       +3

Impacted Files	Coverage Δ
testing/test_reports.py	10.49% <0%> (-89.51%)	⬇️
testing/test_resultlog.py	53.6% <0%> (-46.4%)	⬇️
testing/test_runner.py	59.65% <0%> (-36.62%)	⬇️
testing/python/collect.py	74.2% <0%> (-25.17%)	⬇️
src/_pytest/reports.py	71.42% <0%> (-23.63%)	⬇️
src/_pytest/resultlog.py	67.14% <0%> (-19.43%)	⬇️
testing/test_argcomplete.py	68.11% <0%> (-13.37%)	⬇️
testing/python/fixtures.py	83.96% <0%> (-6.61%)	⬇️
testing/python/integration.py	91.42% <0%> (-2.15%)	⬇️
src/_pytest/assertion/__init__.py	86.76% <0%> (-1.95%)	⬇️
... and 90 more

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[nicoddemus]

Thanks @Pulkit07!

Small tip: If you write "Fix #(num)" in the body of the PR or commit, it will automatically close the related issue when it gets merged. See:

https://help.github.com/en/articles/closing-issues-using-keywords

I've updated the description to use "Fix #(num)".

Also, I just realized that this should target master, as is a small docs update. I've updated the base branch myself.

[nicoddemus]

Merging as docs passed already. Thanks @Pulkit07 again for the PR. 👍