update docs

[teovin]

This PR merges the developer.md and install.md files and makes a few corrections to wording, URLs, and commands.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.64%. Comparing base (60ead32) to head (c593f57).
⚠️ Report is 22 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3769   +/-   ##
========================================
  Coverage    69.64%   69.64%           
========================================
  Files           65       65           
  Lines         7980     7980           
========================================
  Hits          5558     5558           
  Misses        2422     2422           

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[cmsetzer]

Thanks for the editing! This looks like a step forward. A couple other thoughts:

• Can we merge install.md and developer.md into one doc? I get confused about which one to read for instructions when I have to rebuild Perma.

• While we're cleaning up the docs, can we standardize the Markdown code blocks? These docs currently use a mix of:

  ## Single backticks on own line:
  
  `d pytest`
  
  ## Indent with CLI prompt:
  
      $ docker compose up -d
  
  ## Fenced code blocks:
  
  ```sh
  d invoke lock --args "--upgrade-package package_name"
  ```

  My opinion: we should always use fenced blocks for non-inline code examples. What do you think?

[teovin]

@cmsetzer I merged the two mds and also standardized the non-inline code blocks and Flake8. Let me know if there is anything else, otherwise, I will merge this.

[cmsetzer]

@teovin Looks good to me, thanks for working on this!