docs: add --edge to install section

[jsjoeio]

This adds documentation for the flag --edge which we've had for a
while but forgot to document.

Fixes #5717

[github-actions]

✨ code-server docs for PR #5767 is ready! It will be updated on every commit.

• Host: https://coder.com/docs/code-server/50c3e85
• Last deploy status: success
• Commit: a67f1e9
• Workflow status: https://github.com/coder/code-server/actions/runs/3432231251

[github-actions]

✨ code-server dev build published to npm for PR #5767!

• Last publish status: success
• Commit: a67f1e9

To install in a local project, run:

npm install @coder/code-server-pr@5767

To install globally, run:

npm install -g @coder/code-server-pr@5767

[codecov]

Codecov Report

Merging #5767 (a67f1e9) into main (3367e1a) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5767   +/-   ##
=======================================
  Coverage   72.66%   72.66%           
=======================================
  Files          30       30           
  Lines        1683     1683           
  Branches      370      370           
=======================================
  Hits         1223     1223           
  Misses        397      397           
  Partials       63       63           

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