Add details of purgecss to one of the docs pages - maybe https://katex.org/docs/issues.html

(Partly) relates to performance issues in https://github.com/KaTeX/KaTeX/issues/2194

**Is your feature request related to a problem? Please describe.**
I was able to reduce my katex.min.css from 23KB to 12KB by using [purgecss](https://purgecss.com/CLI.html).
This is particularly useful because I inline this into my `` as it doesn't really make sense to flash unstyled equations at people because the equations are the meat of the content.

**Describe the solution you'd like:**
Since KaTeX is layout/style/rendering intensive, it may be good to highlight this tool on the docs pages.

I suggested https://katex.org/docs/issues.html, but maybe we need a section for "optimising KaTeX"? I have some things to say about glyphhanger, too (I'll write a discussion thread about that in a moment).

I'll probably get round to writing a pull request eventually, but I won't be at all upset if someone wants to do this before I get to it!

I guess that as well as reducing the number of bytes there is a small performance improvement here, which I could document if I get the time.