Support webtex for Github documents format

[cderv]

This should resolve #1886 and #2003

This adds a math_method argument to github_document() so that equation can be rendered as an image in the resulting md document

---
output:
  github_document:
    math_method: "webtex"
---

Some content

$$
\int \frac{1}{x} dx = \ln \left| x \right| + C
$$

is rendered as

---

Some content

---

We ended up activating by default in github document where there is math. Like in Quarto.

We default to using PNG format with a DPI of 110 and using white background because otherwise (using svg for example) the equation would not be seen on dark mode in Github. We can't change the color font with the API.
However, Quarto defaults to SVG because this gives way cleaner output. Quarto documents how to change the background using PNG. Quarto doc about this: https://quarto.org/docs/output-formats/gfm.html#webtex-math

Should we default to the same ?

One thing is I am not sure about how we should consider CodeCog fair usage policy. https://editor.codecogs.com/docs/3-fair_usage.php, but I guess it is ok.

[yihui]

Looks great. Thanks!