Extract H3.0 license into its own content/ folder, update documentation

[nilsnh]

Milestone

Over time I see that the HL3 license files differ more and more from the other licenses inside the version/ folder. The previous versions have code in /layout/version/* that is particular to them but not to the HL3.

To better disambiguate the differences in how the licenses are built I moved the HL3 to it's own folder _build-dependency-hl-3.0/ and created a corresponding layout file in /layout/_build-dependency-hl-3.0/.

This PR does not change the HL 3.0 license URL /version/3/0/full.html because that is handled by the Netlify function and proxy defined in netlify.toml.

Also, the PR fixes the Hugo build warning where it complains about missing layout plaintext / markdown / asciidoc file for kind "page".

Finally this PR documents how the HL3 license is built in the README.md

Getting there

• Fixed Hugo build warning.
• Thought a lot about a better name for the HL3.0 builder folder, and tried different variations before finally settling.
• Updated the documentation afterwards.

Interesting bits

• We delete sub-folder /public/_build-dependency-hl-3.0/ after deployment because:
  • prepareLambdas.mjs converts the full license text to JSON to be used by the Netlify function.
  • Also, Hugo has then finished importing the license source into the /build page.
  • It's not a risk to share the built folder /public/_build-dependency-hl-3.0/, but we delete it at the end of the build defined in netlify.toml so we don't have to prevent it from being indexed by Google with a robots.txt.

What I need help with

• I'm pretty happy with this change, but do chime in if there is a better change I haven't considered or if there are stumbling blocks with this I haven't considered.

What's next

After merging, this PR should allow us to:

• More easily add other contributors.
• More easily work on all past and future versions of the HL license.

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