chore(ci): only run docs preview on docs changes

[jsjoeio]

This speeds up CI slightly.

[github-actions]

✨ code-server dev build published to npm for PR #5368!

• Last publish status: success
• Commit: c9c43b6

To install in a local project, run:

npm install @coder/code-server-pr@5368

To install globally, run:

npm install -g @coder/code-server-pr@5368

[codecov]

Codecov Report

Merging #5368 (c9c43b6) into main (40b650b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #5368   +/-   ##
=======================================
  Coverage   72.42%   72.42%           
=======================================
  Files          30       30           
  Lines        1672     1672           
  Branches      366      366           
=======================================
  Hits         1211     1211           
  Misses        398      398           
  Partials       63       63           

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