Document SVG 'paint server'

[bsmth]

We're referencing "paint servers" in SVG docs (and soon in CSS props that override attrs). We need some disambiguation so this terminology is not confused with other types of servers.

Either we have a page about paint servers under SVG, or we have a Glossary entry to start.

Docs that mention 'paint server':

• https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/fill
• https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/fill-opacity
• https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/stroke
• https://developer.mozilla.org/en-US/docs/Web/SVG/Element#paint_server_elements

spec link: https://www.w3.org/TR/SVG2/pservers.html

[techbabe23]

Hi, I would like to work on this issue!