chore: add docs preview workflow

[BrunoQuaresma]

Add a new workflow to generate docs preview. It is the same workflow used by Coder docs with some minor changes.

[github-actions]

✨ Coder.com for PR #3958 deployed! It will be updated on every commit.

• Host: https://codercom-r41d15q9a-codercom.vercel.app/docs/code-server
• Last deploy status: success
• Commit: 90e6310
• Workflow status: https://github.com/cdr/code-server/actions/runs/1121135598

[jsjoeio]

Wow! This is awesome! 🙌

This makes it 1000x easier to preview new doc changes. Thanks @BrunoQuaresma ♥️

[codecov]

Codecov Report

Merging #3958 (31373d8) into main (be0e5de) will increase coverage by 0.90%.
The diff coverage is 100.00%.

❗ Current head 31373d8 differs from pull request most recent head 90e6310. Consider uploading reports for the commit 90e6310 to get more accurate results

@@            Coverage Diff             @@
##             main    #3958      +/-   ##
==========================================
+ Coverage   62.60%   63.51%   +0.90%     
==========================================
  Files          36       36              
  Lines        1872     1872              
  Branches      379      379              
==========================================
+ Hits         1172     1189      +17     
+ Misses        595      580      -15     
+ Partials      105      103       -2     

Impacted Files	Coverage Δ
src/browser/register.ts	100.00% <100.00%> (ø)
src/node/proxy_agent.ts	60.00% <100.00%> (+60.00%)	⬆️
src/node/proxy.ts	100.00% <0.00%> (+22.22%)	⬆️

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[jsjoeio]

will increase coverage by 0.90%.

Totally unsure how this happened but won't complain 😄

[BrunoQuaresma]

will increase coverage by 0.90%.

Totally unsure how this happened but won't complain 😄

LOL, me neither. My guess is the coverage is being calculated, including all the files. Maybe we can improve this one by excluding extensions unrelated to the app itsewlf, like .ymal and .md