Add internal link checker

[pedro-psb]

Although link checking will not be strictly static (it may try to build the site), I'd like to put it under a general linting category of commands. For now, the only "linter" would be internal link checker (should we add external as well?)

# run all linting on all code
pulp-docs lint

# run link-check on all code
pulp-docs lint --linters link-check

# run link-check on pulp-docs only
pulp-docs lint --linters link-check --repos pulp-docs

Approaches Considered

• Leverage mkdocs validation + tricks to select only links from a single repository
• Find another link checker