Refactor build gh-action to use Makefile and embed images into HTML document

[cofinoa]

See issue #523 for discussion of these changes.

Release checklist

• Authors updated in cf-conventions.adoc? Add in two places: on line 3 and under .Additional Authors in About the authors.
• [NA] Next version in cf-conventions.adoc up to date? Versioning inspired by SemVer.
• history.adoc up to date?
• [NA ] Conformance document up to date?

For maintainers

After the merge remember to delete the source branch.
Tags are set at the conclusion of the annual meeting; until then, main always is a draft for the next version.

[larsbarring]

To my pretty limited knowledge this looks fine.

[cofinoa]

@JonathanGregory and @davidhassell

I know that we are in the process to publish 1.12, but I think this PR could contribute to improve the "user" experience to build the conventions and conformance documents.

[cofinoa]

@davidhassell and @JonathanGregory

I have added and updated the Makefile to build the conformance and conventions documents.

The Makefile allows building just one document and format (or both), but only in your local computer.

I have updated the README.md accordingly, please review and update.

I'm modifying the GH action to use the Makefile in the build process.