Replace `<a>` with Markdown links

This cleans up the Markdown and ensures no HTML slips into content and
corrupts rendering. CSS attribute selectors now apply the same
Octicons, resulting in no visual change to the page.
Finalize Workbook link attribute-specific styling

Isolate the Octicon-styled links to only those within the `<section>`
scoped within the class selector “workbook” to prevent any other links
on page receiving icons.
Remove remaining `<a>`, replace with Markdown link

Ensures the Markdown files are 100% only Markdown without any HTML tags
Merge pull request #99 from github/workbook-tune-up

Remove all `<a>` in Workbook Markdown