docs: remove markdown

[almeidx]

Please describe the changes this PR makes and why it should be merged:
The documentation website does not support rendering markdown

Status and versioning classification:

• This PR only includes non-code changes, like changes to documentation, README, etc.

[vercel]

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2 Ignored Deployments

Name	Status	Preview	Comments	Updated (UTC)
discord-js	⬜️ Ignored (Inspect)	Visit Preview		Nov 5, 2023 6:16pm
discord-js-guide	⬜️ Ignored (Inspect)	Visit Preview		Nov 5, 2023 6:16pm

[codecov]

Codecov Report

Merging #9914 (1ebbcfc) into main (e80194e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #9914   +/-   ##
=======================================
  Coverage   60.04%   60.04%           
=======================================
  Files         235      235           
  Lines       16027    16027           
  Branches     1216     1216           
=======================================
  Hits         9624     9624           
  Misses       6359     6359           
  Partials       44       44           

Flag	Coverage Δ
builders	98.69% <ø> (ø)
formatters	99.45% <100.00%> (ø)
next	∅ <ø> (∅)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
packages/formatters/src/formatters.ts	100.00% <100.00%> (ø)

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	69
🟢 Accessibility	96
🟢 Best practices	100
🟠 SEO	67
🔴 PWA	33

Lighthouse ran on https://discord-js-git-docs-remove-markdown-discordjs.vercel.app/