Lingering and future work for `nbmail`

[juleswg23]

These are some leftover tidbits from my work with nbmail that might be nice to tackle, including some nits from #24 that I haven't quite cleaned up.

• Tests: missing coverage in compose module, as well as Email.show_browser() and _process_tag(). Im not particularly worried about testing the mjml/tags.py module, since they are all MJMLTag objects, which is tested in mjml/_core.py.
• Check for support of inline tables passed with GT.as_raw_html(inline_css=True), and improve rendering if need be. I suspect this will work in a block_text() without much trouble.
• Quarto gives this warning WARNING (/Applications/quarto/share/filters/main.lua:9319) Unable to parse table from raw html block: skipping. when rendering the /reference/compose.compose_email.html generated from quartodoc. This also occurs with other email __repr__() functions. I suspect it has to do with the MJML email content including table divs as a means of displaying nicely in email clients.
• Possibly similar to above, but multiple blocks does not render as expected in the documentation. More detail can be found in Documentation renders email with unexpected gaps #27
• egress.py may want more support. A lot of work on authoring has been done in this package, but the sending behavior (unless offloaded to Connect or another service) is probably better suited for personal use at the moment.
• Convert urls that used to be https://posit-dev.github.io/email-for-data-science/* to https://posit-dev.github.io/nbmail/*