theme: Add link to Mastodon

Closes #3073

Make sponsor logo responsive

theme: Adjust sponsor

theme: Add current Hugo version to the header

* Then I don't have to check the source to see if we're at latest.
* Also, it has some promo value.

theme: Misc adjustments for the themes site

netlify: Hugo 0.146.3

hugoreleaser.toml => hugoreleaser.yaml

Describe how to configure uglyURLs per section

netlify: Hugo 0.128.2

Update description of Markdown content format