Add hugo version check and theme update to Makefile

[nginx-jack]

Proposed changes

This should help prevent any hugo or hugo-theme version drift going forward, and warn when versions are out of date.

• Add hugo version check and theme update to Makefile
• Update README.md with correct make target behaviors
• Add hugo-entrypoint to allow extra commands when docker starts
• Update hugo theme version in go.mod
• Use slimmer hugo docker image

Closes #2544

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• [x] I have read the CONTRIBUTING doc
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have checked that all unit tests pass after adding my changes
• [x] I have updated necessary documentation
• [x] I have rebased my branch onto main
• [x] I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes, please add a brief note that summarizes the change.

[nginx-bot[bot]]

Hi @nginx-jack!

Thanks for opening this pull request! Be sure to check out our Contributing Guidelines while you wait for someone on the team to review this.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.62%. Comparing base (1294e2d) to head (a6204fe). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2608 +/- ## ======================================= Coverage 88.62% 88.62% ======================================= Files 105 105 Lines 8116 8116 Branches 50 50 ======================================= Hits 7193 7193 Misses 866 866 Partials 57 57 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.