Update readme and create sample env file for local dev

[baarkerlounger]

Description of change

Make it easier to get a fully local (non-dockerised) setup up and running

• Clarifies and updates readme steps
• Separates sample env files into docker and local dev specific ones for easier startup
  • This has the downside of needing to maintain 2 files when we add or remove env vars but this usually happens quite infrequently
  • The upside is simpler setup for new comers

Checklist

• Has this branch been rebased on top of the current main branch?

  Explanation

  The branch should not be stale or have conflicts at the time reviews are requested.

• Is the CircleCI build passing?

General points

Other things to check

• Make sure fixtures/test_data.yaml is maintained when updating models
• Consider the admin site when making changes to models
• Use select-/prefetch-related field lists in views and search apps, and update them when fields are added
• Make sure the README is updated e.g. when adding new environment variables

See docs/CONTRIBUTING.md for more guidelines.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.36%. Comparing base (d08d132) to head (6c54ca0).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5651      +/-   ##
==========================================
- Coverage   96.37%   96.36%   -0.01%     
==========================================
  Files        1010     1010              
  Lines       23939    23939              
  Branches     2153     2153              
==========================================
- Hits        23071    23069       -2     
- Misses        702      703       +1     
- Partials      166      167       +1     

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