Integrate HTML docs into extension build

[jreineckearm]

A workflow has been added to build HTML docs from MD files in this repo.

At the moment the build output is "only" published to the GitHub pages. But we also need to include a copy into the extension build. We then should add the source MD files to the .vscodeignore list.

One idea to expose the docs is through the CMSIS outline view. Will prepare a draft to further discuss that.

[ReinhardKeil]

The easiest way to solve this is a common manual for all CMSIS related features that is accessed via the CMSIS View ... Help

[jreineckearm]

See screenshots in draft #245 . Happy to continue that if approach acceptable.

[jreineckearm]

Approach changed, light-weight user documentation now in README.MD. No more plans to include mkdocs output into extension.