Small re-organization of documentation files

[The-Mule]

Description

Currently, there are multiple documentation files that are not connected in any way. This PR adds a brief project description (previously split into two places), licence badge and links to all the other documentation files (BUILD.md and HOWTO.md). Small re-formatting was also applied to BUILD.md and HOWTO.md without removing existing or adding any new content.

I considered adding more examples to HOWTO about key management. But once you configure openssl to use the provider (covered in HOWTO.md) the rest is really more about other tools (e.g. p11-kit) than the provider.

I also did not cover tests but we can do that later if needed.

Resolves #301.

Checklist

Not applicable

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• This feature/change has adequate documentation added
• Code conform to coding style that today cannot yet be enforced via the check style test
• Commits have short titles and sensible commit messages
• Coverity Scan has run if needed (code PR) and no new defects were found

[The-Mule]

Converted to draft to add a HOWTO note related to #303 (see the last comment there).