chore: Validate HTML on PR commits

[jbmoelker]

Changes

• Adds lint:html script that validates production build HTML.
• Runs script on PR commits.
• Fixes current HTML violations.

Associated issue

Resolves #6

How to test

• Checkout this branch (chore/validate-html).
• Create an HTML violation in a .astro file.
• Create a production build: npm run build.
• Validate the output HTML: npm run lint:html.
• Review GitHub Action runs on this PR
• Verify lint:html throws errors on initial commit:

Run npm run lint:html

> head-start@0.0.1 lint:html
> html-validate --config config/htmlvalidate/config.json dist/

...

Error:    1:4170  error  End tag for <meta> must be omitted              void-content
Error:    3:635   error  Redundant role "list" on <ul>                   no-redundant-role
Error:   12:2346  error  <iframe> is missing required "title" attribute  element-required-attributes

• Verify it passes after fixes were made in second commit.

Checklist

• [x] I have performed a self-review of my own code
• [x] I have made sure that my PR is easy to review (not too big, includes comments)
• I have made updated relevant documentation files (in project README, docs/, etc)
• I have added a decision log entry if the change affects the architecture or changes a significant technology
• [x] I have notified a reviewer

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