Add a 'skip to content' link for accessibility

[freephile]

https://www.w3.org/TR/WCAG20-TECHS/G1.html

Example: Vector Skin https://gerrit.wikimedia.org/g/mediawiki/skins/Vector/+/dd8cc5dfe80a16d6f0013e2ba7c9c7c0a3c897b4/includes/templates/skin.mustache#35

[freephile]

I will try to do this in my copious spare time, unless someone can beat me to it.