Add a hidden skip to main link

[neal-timpe]

The automated test Felicia ran with the IBM Accessibility checker, whose results are posted in this work item, that shows there needs to be a skip to main link on this page.

The screen reader reads "Red Hat supplementary style guide for product documentation"

I confirmed that with a screenreader test with VoiceOver on MacOS.

Then, the screanreader reads the entire navigation before they can interact with the content.

Here is WCAG item 2.4.1 that explains this issue.

And a link to the techniques page.