Add a guide on prebuilds

[bamurtaugh]

Fixes devcontainers/spec#261.

@craiglpeters I updated the info on the Kubernetes example a bit based on our chat last week - please feel free to propose other changes, or add in other tips/feedback from your experience working on the prebuild!

I also updated post.html to hopefully allow for 2 author posts correctly - cc @joshspicer in case you have any feedback on a different way to achieve this.

cc @olguzzar thanks again for the feedback - I'd love an additional review as I've incorporated your feedback and made a few more updates! I also created https://aka.ms/dev-container-prebuild that'll point to this guide for us to use in any docs or advocacy work.

[samruddhikhandale]

This is awesome and pretty helpful!
Thank you, Brigit and Craig 👏 ✨

[joshspicer]

I opened this PR up in a codespace (with bundle exec jeykll serve) and i'm not seeing the authors anymore 🤔

[joshspicer]

Discussed my comment on authors with Brigit offline :)

[olguzzar]

This looks so great Brigit!!

[bamurtaugh]

Incorporated feedback from yesterday's discussion (cross-link to this post from Chuck's guide on images vs dockerfile, added info on multi-stage builds). Would love another quick set of reviews, then I think we can merge this in and of course keep iterating in the future!

[chrmarti]

Looks great. Maybe a separate sample repo just showing off prebuilds would makes it easier for users to get an overview of what is needed. Not a requirement though.

[bamurtaugh]

Thanks so much @chrmarti! That's great feedback, and I've opened a tracking issue (in case anyone from the community happens to have one they'd like to contribute too!): #281.