docs: improve hx-vals documentation for use with hx-post

[yujiteshima]

Description

Improve the hx-vals documentation to address common user confusion, especially regarding its use with hx-post and other non-GET HTTP methods.

Changes made:

1. Added "Using hx-vals with hx-post" section

   • Example showing hx-vals with hx-post
   • Clarified that hx-vals works with all HTTP methods (POST, PUT, PATCH, DELETE)
   • Explained the difference: non-GET requests include values in the request body, while GET requests append them as query parameters

2. Added "JSON Syntax Requirements" section

   • Documented a common pitfall: using single quotes inside JSON ({'key': 'value'}) which silently fails
   • Provided correct examples using double quotes inside JSON
   • Warned that malformed JSON is silently ignored, making debugging difficult

These additions address user feedback in issue #1134, where multiple users reported spending significant time debugging issues that could have been avoided with clearer documentation.

Corresponding issue: #1134

Testing

This is a documentation-only change. No functional code was modified.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded