Mention fig.alt in the docs

[hturner]

I'm happy to see that fig.alt is supported by litedown, but there is no mention of it in the docs. Since this was not always possible with rmarkdown I think it is worth including in an example or mentioning explicitly somewhere.

[yihui]

Yes, the documentation needs a lot more work (which may take two months). Not only fig.alt, but most chunk options are not documented yet.