docs: remove dead Job Board links

[Jerry-CodeHub]

Checklist

• I have ensured my pull request is not behind the main or master branch of the original repository.
• I have rebased all commits where necessary so that reviewing this pull request can be done without having to merge it first.
• I have written a commit message that passes commitlint linting.
• I have ensured that my code changes pass linting tests.
• I have ensured that my code changes pass unit tests.
• I have described my pull request and the reasons for code changes along with context if necessary.

Summary

• Remove the "Job Board" section from README as all links are dead (404)

Details

The astro.netlify.com service was added by TJ Holowaychuk in April 2017 as a
private job board sponsorship service for open source projects. This service is
no longer operational - all URLs return 404 errors.

The affected links pointed to:

• Automattic
• Segment
• Auth0

The Backers and Sponsors sections using OpenCollective remain functional and are preserved.

Test plan

• Verified all astro.netlify.com URLs return 404
• Confirmed OpenCollective links still work

Issues: [docs] Job Board three link and image not show in readme #1911

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.90%. Comparing base (fecd464) to head (3e889e4).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1926   +/-   ##
=======================================
  Coverage   99.90%   99.90%           
=======================================
  Files           9        9           
  Lines        2084     2084           
=======================================
  Hits         2082     2082           
  Misses          2        2           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.