This commit replaces some outdated URLs in "Contribute" links (on the
"Contribute" page):
- `github.com/angular/material2` --> `github.com/angular/components`
- `github.com/angular/angularfire2` --> `github.com/angular/angularfire`
It also replaces the deprecated `md-button` attribute with `mat-button`.
PR Close#40944
In the marketing pages we do not want to show heading anchors on hover.
Previously, this was achieved by using div rather than heading elements.
Now we can use semantically accurate headings while hiding the anchor.
Closes#17244Closes#17264