Powershell: Add CI for validating links in Markdown files

[andschwa]

With, say, awesome_bot.

[andschwa]

I think what we're looking for here is:

• [ ] ensuring links are correct (not broken)
• [x] spell checking of markdown files
• [x] maybe style checking

Since AppVeyor is consistently faster than Travis CI, and this probably only needs to be in one place, an extra step only on AppVeyor I think would be sufficient.

@vors started creating a PowerShell-based MarkdownLinkChecker that could be used, but IIRC it couldn't handle checking relative links [example](../docs/example), which is actually our primary link use case.

I think this issue would be a good Hacktoberfest submission.

[vors]

Actually it's the exact oposite: MarkdownLinkCheck checks relative links fine and doesn't check absolute links

[iSazonov]

spell checking of markdown files

Solved.

[joeyaiello]

Two out of three of the above are fixed, the only part that we still need is link validation.

@TravisEz13 could you please unassign if you're not actively working on the hyperlink aspect (which is not a 6.0.0 priority)?

[RDIL]

Completed by me. Please close @TravisEz13 or @SteveL-MSFT

[TravisEz13]

@RDIL thanks again. Did you review my work to change yours into a pester test?

[RDIL]

Um if it was already merged then yes I believe so.