add mdv to validate markdown links

[shanejonas]

Is your feature request related to a problem? Please describe.
I'm always frustrated when markdown links are broken. They are a big part of markdown and specs.

Describe the solution you'd like
Add mermade/mdv to Jenkinsfile as a step/stage test to validate markdown links on pull requests.

Additional context
see also open-rpc/spec#74

[BelfordZ]

We should also add markdownlint similar to open-rpc/spec#149