Currently contribution guide does not reflect actual flow to raise a new PR and hence it's not clear (for a new contributors) what exactly needs to be done to make a PR for Spark repository and test it as per expectation. This PR addresses that by following:

• It describes in the Pull request section of the Contributing page the actual procedure and takes a contributor through a step by step process.
• It removes optional "Running tests in your forked repository" section on Developer Tools page which is obsolete now and doesn't reflect reality anymore i.e. it says we can test by clicking “Run workflow” button which is not available anymore as workflow does not use "workflow_dispatch" event trigger anymore and was removed in [SPARK-35048][INFRA] Distribute GitHub Actions workflows to fork repositories to share the resources spark#32092
• Instead it documents the new procedure that above PR introduced i.e. contributors needs to use their own GitHub free workflow credits to test new changes they are purposing and a Spark Actions workflow will expect that to be completed before marking PR to be ready for a review.
• Some general wording was copied from "Running tests in your forked repository" section on Developer Tools page but main content was rewritten to meet objective
• Also fixed URL to developer-tools.html to be resolved by parser (that converted it into relative URI) instead of using hard coded absolute URL.

Tested imperically with bundle exec jekyll serve and static files were generated with bundle exec jekyll build commands

This closes https://issues.apache.org/jira/browse/SPARK-37996