Docs: Add example customizing markdown renderer of ui.markdown()

[gadenbuie]

In answering a question on Discord, I came up with the following example that uses commonmark markdown rendering with the attributes plugin enabled.

from shiny.express import ui

from markdown_it import MarkdownIt
from mdit_py_plugins import attrs
md = MarkdownIt().use(attrs.attrs_plugin)

ui.markdown(
    '[shiny](https://shiny.posit.co/py){target="_blank"}',
    render_func=md.render
)

Without the additional customization, the default markdown format of ui.markdown() uses GFM, which doesn't support [link](target){attr="value"} syntax.

[gadenbuie]

We should also document that "gfm" is commonmark with the table, linkify, strikethrough and tasklist plugins enabled (if linkify and tasklist are available).

py-shiny/shiny/ui/_markdown.py

Lines 76 to 84 in 2c2b075

	# Inspired by MyST-Parser's gfm-only option
	# https://github.com/executablebooks/MyST-Parser/blob/ce1245b25/myst_parser/main.py#L257-L269
	parser = MarkdownIt("commonmark", {"linkify": True})
	parser.enable(["table", "linkify", "strikethrough"])
	try:
	from mdit_py_plugins.tasklists import tasklists_plugin
	parser.use(tasklists_plugin) # type: ignore