Add basic examples in Starter Template

[mbourne]

It's been a long while since I used KaTeX "from scratch" so I made use of the Starter Template on this page: https://katex.org/docs/browser.html

I placed some equations in the body and loaded the page, but it just showed the equations as text. It turned out it was the issue I reported on the default delimiters - the environments I was using happened to not be in that default list. There was no error message since KaTeX just thought the page was devoid of math and I was stumped until I tried a few $$ ... $$ equations, which did work OK.

It would have helped if the Starter Template was a bit more extensive and included a body with some examples that will actually work out of the box, and a note that if other environments were required, to add an options object, as per https://katex.org/docs/autorender.html (or, if all supported environments were included in the defaults, as I mentioned.)

This will help people who are brand new to the library, I believe.

I'm sorry if this is documented somewhere and I missed it.