Support KaTeX in help page viewer

[renkun-ken]

What problem did you solve?

Closes #1207

In the HTML of R help pages, the katex JS/CSS resource urls are transformed so that they can be properly loaded in the webview:

<link rel="stylesheet" href="/doc/html/katex/katex.css">
<script type="text/javascript" src="/doc/html/katex-config.js"></script>
<script defer="" src="/doc/html/katex/katex.js" onload="processMathHTML();"></script>

these are replaced with

<link rel="stylesheet" href="http://localhost:<port>/doc/html/katex/katex.css">
<script type="text/javascript" src="http://localhost:<port>/doc/html/katex-config.js"></script>
<script defer="" src="http://localhost:<port>/doc/html/katex/katex.js" onload="processMathHTML();"></script>

where http://localhost:<port> is provided by the R help server running in the background.

(If you have)Screenshot

(If you do not have screenshot) How can I check this pull request?

Open a help page and see if the katex formulas are properly rendered.

?dt

[renkun-ken]

Looks like it does not work under remote development yet.

I'll take a closer look at this later.