docs: add custom preset instructions

[Hebilicious]

🔗 Linked issue

Resolves #1373

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Adds some instructions about custom presets.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[codecov]

Codecov Report

Merging #1409 (7913e74) into main (6bc50b2) will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1409      +/-   ##
==========================================
- Coverage   76.73%   76.68%   -0.06%     
==========================================
  Files          71       71              
  Lines        7273     7273              
  Branches      727      726       -1     
==========================================
- Hits         5581     5577       -4     
- Misses       1691     1694       +3     
- Partials        1        2       +1     

see 1 file with indirect coverage changes

[pi0]

Starter is updated: https://github.com/unjs/nitro-preset-starter

As per discussion, we shall point the users from docs to this repo for having latest version in same place.

[Hebilicious]

I've updated the PR with a link to the new starter

[pi0]

Thanks for helping on this ❤️