Document MkDocs native support for anchor validation

[johnthagen]

Closes #84

[manuzhang]

@johnthagen Does this mean we can remove checks for anchors?

[johnthagen]

@manuzhang I'm not sure that we should remove this packages anchor validation just yet, given the the MkDocs anchor validation is new and has some known issues with some plugins

• Anchor validation false positives after upgrading to 1.6 mkdocs/mkdocs#3690

For now, I think it's best to raise awareness for this new feature in MkDocs and after it has matured, we could consider removing anchor validation from this library.