docs: add a2a-php to community SDKs

[praveendias1180]

Description

Adds a2a-php (PHP SDK for A2A v1.0.0) to the Community SDKs section of docs/community.md, as requested in #2262.

• Entry follows the existing format (heading, badge line, one-line summary) and is appended after the C++ entry.
• Adds laravel and praveendias (the GitHub owner in the URLs) to .github/actions/spelling/allow.txt, kept in LC_ALL=C sort order as scripts/sort_spelling.sh does.

About the SDK, against the listed requirements:

• Spec compliance: official A2A TCK in CI, MUST level 0 failures over JSON-RPC and HTTP+JSON (161 distinct MUST tests), SHOULD 13/13, MAY 7/7: https://praveendias1180.github.io/a2a-php/reference/conformance/
• Published package: https://packagist.org/packages/praveendias1180/a2a-php (v1.0.0), plus a Laravel bridge https://packagist.org/packages/praveendias1180/a2a-laravel
• Documentation: https://praveendias1180.github.io/a2a-php/ (guides, API reference, Python SDK mapping)
• Tests with CI: https://github.com/praveendias1180/a2a-php/actions/workflows/ci.yml (unit tests on PHP 8.2 to 8.5, TCK, interop with the official Python SDK in both directions)
• Apache 2.0, actively maintained.

Checklist:

• Follow the CONTRIBUTING Guide.
• Make your Pull Request title follow the Conventional Commits specification and repository rules (docs: for other docs).
• Ensure the tests and linter pass (scripts/sort_spelling.sh leaves the allow list unchanged; markdownlint docs/community.md --config .github/linters/.markdownlint.json passes).
• Appropriate docs were updated (if necessary)

Fixes #2262 🦕