Search API Documentation

[MRichards99]

This PR will close #320, #328

Description

This PR updates the documentation to give details of the search API. Along the way, I've cleaned up a few existing things and removed aspects that are difficult to regularly update/get outdated rather quickly (e.g. contents, directory tree).

I've tried to include important bits of how the search API works, including details of how the mapping file works and the simple client handling (compared to DataGateway API anyway).

I've also moved the Postman collection to util/ because the documentation implies that's where it lives, and it seems like a sensible place for it.

No version bump on this PR as there are no code changes.

Testing Instructions

I mean, can you even test a markdown file 🤔

• Review code
• Check GitHub Actions build
• If icatdb Generator Script Consistency Test CI job fails, is this because of a deliberate change made to the script to change generated data (which isn't actually a problem) or is here an underlying issue with the changes made?
• Review changes to test coverage
• Does this change mean a new patch, minor or major version should be made? If so, does one of the commit messages feature fix:, feat: or BREAKING CHANGE: so a release is automatically made via GitHub Actions upon merge?

Agile Board Tracking

Connect to #328

[codecov]

Codecov Report

Merging #333 (0a9c05a) into master (11d59e8) will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #333      +/-   ##
==========================================
+ Coverage   92.62%   92.69%   +0.06%     
==========================================
  Files          43       43              
  Lines        3215     3216       +1     
  Branches      321      321              
==========================================
+ Hits         2978     2981       +3     
  Misses        198      198              
+ Partials       39       37       -2     

Impacted Files	Coverage Δ
datagateway_api/src/search_api/helpers.py	97.46% <0.00%> (-2.54%)	⬇️
...gateway_api/src/search_api/query_filter_factory.py	95.23% <0.00%> (-0.32%)	⬇️
datagateway_api/src/common/config.py	100.00% <0.00%> (ø)
...agateway_api/src/resources/search_api_endpoints.py	100.00% <0.00%> (ø)
datagateway_api/src/api_start_utils.py	86.29% <0.00%> (+0.45%)	⬆️
datagateway_api/src/search_api/models.py	98.50% <0.00%> (+1.80%)	⬆️

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