Export documentation to PDF

[rexwangcc]

It's a huge plus if we can let users export documentation to PDF files so they don't have to be online to check the docs.

Note: This can happen at either build time or in-broswer, depends on which is easier to implement.

[w-m]

And it would be very useful to feed the complete documentation as context to an LLM, when trying to ask about or write taichi code. (I came to this issues looking for a documentation PDF to do that).