Change from root-relative to absolute path for documentation in README. Update link for CI status badge.

[jcurtis2]

• Changed CI status badge link to a new link format as old CI status badge link led to "No Status".
• Changed to absolute path for documentation links as root-relative paths worked as intended online (https://compdyn.github.io/partmc/) but obviously fail either locally and/or on GitHub page.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.50%. Comparing base (1e67b40) to head (4376889).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #220   +/-   ##
=======================================
  Coverage   76.50%   76.50%           
=======================================
  Files          54       54           
  Lines        9118     9118           
=======================================
  Hits         6976     6976           
  Misses       2142     2142           

☔ 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.

[slayoo]

thanks!