docs(deployments): add Vercel deployment guide (#390)

[echonesis]

Changes

Adding the first part of #390 — deploying Burr applications to Vercel (serverless, one-click).

This PR introduces:

• examples/deployment/vercel/README.md with step-by-step instructions
• POST test examples

How I tested this

Tested & verified on a free Vercel account: https://test-burr-vercel.vercel.app

Demo request works:

curl -X POST https://test-burr-vercel.vercel.app/api/counter \
  -H "Content-Type: application/json" \
  -d '{"number": 5}'

Notes

Developers could use 2 methods:

• Vercel CLI
• GitHub Link

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.

N/A: (Since this PR is used for deployment, no functionality changes are included.)

• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.

[skrawcz]

@echonesis thanks for adding this. How can I validate this? Unfortunately the app isn't up anymore.

[skrawcz]

can you add to the docs/ as well a link to this -- i.e. edit this page

[echonesis]

Yes, I will update it in the next commit.

[skrawcz]

cool, I think that's all that's really needed for this to be ready.

[echonesis]

@echonesis thanks for adding this. How can I validate this? Unfortunately the app isn't up anymore.

Thanks @skrawcz
Currently I only provide POST API, just like the AWS deployment example.

[skrawcz]

is there a way to stream results back? doesn't have to be this commit, but I think it's a good way to show case something happening.

[skrawcz]

@echonesis thanks for adding this. How can I validate this? Unfortunately the app isn't up anymore.

Thanks @skrawcz Currently I only provide POST API, just like the AWS deployment example.

oh yep -- it works. thanks. validated.