Add more PDF and video resources

[melissawm]

Includes

• Megablocks slides and video link
• Intro to JAX and TPUs slides and video link

This PR is split from #266 to simplify review.

[melissawm]

Hi all, gentle ping here. Any feedback I can address? Thanks!

[emilyfertig]

I think we should avoid adding subheadings to the navigation bar (e.g. Intro to JAX & TPUs, Megablocks) that only contain a sentence or two with links to other material. All of the other subheadings link to longer examples or tutorials. I would propose adding a "Learning" subheading under "Other Resources" and listing Intro to JAX & TPUs and Megablocks there (and moving the content of the "More Learning Resources" section that's currently under "Getting Started" to there as well).

[melissawm]

Thank @emilyfertig - updated! Let me know if this is better.

[emilyfertig]

Sorry I had missed that this PR added PDF files -- I think we want to try to avoid this, to minimize the size of the repo when people clone it (even if we delete the PDFs later, my understanding is they'll always be downloaded when the repo is cloned). Could we remove the PDF files and link to them somewhere else, or if there's no other link, just omit it and only link to the videos?