docs: add how to change port

[jsjoeio]

Fixes #3957

[codecov]

Codecov Report

Merging #4031 (6f8dba7) into main (1d8806f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #4031   +/-   ##
=======================================
  Coverage   63.51%   63.51%           
=======================================
  Files          36       36           
  Lines        1872     1872           
  Branches      379      379           
=======================================
  Hits         1189     1189           
  Misses        580      580           
  Partials      103      103           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1d8806f...6f8dba7. Read the comment docs.

[github-actions]

✨ Coder.com for PR #4031 deployed! It will be updated on every commit.

• Host: https://codercom-i7au6tnwk-codercom.vercel.app/docs/code-server
• Last deploy status: success
• Commit: 6f8dba7
• Workflow status: https://github.com/cdr/code-server/actions/runs/1164369228