Add support for table caption

Provides styling of table caption element
Provides more configuration of side nav

Adds a few more variables to configure headings and link hover background
Allow controlling heading sizes on small screens

Introduced variables $h1-font-reduction etc to allow customizing font sizes on mobile separately without overriding them in a custom stylesheet.
Merge pull request #5170 from danielck/master

Allow controlling heading sizes on small screens, fixes #5169
Merge pull request #5033 from clyates/patch-5

Provides more configuration of side nav
Merge pull request #5032 from clyates/patch-4

Add support for table caption
adfix for overriding styles on subheader
undo PR 5128 as it was a breakig chnage
Changed reveal top position