Add architecture overview.

[brainwane]

Work in progress to add an architecture overview to pip, based on the notes @pradyunsg and I took at PyCon North America 2019, similar to Warehouse's overview.

[brainwane]

Thanks to @ppiyakk2 and @dwighthubbard for questions and feedback!

I lightly edited the rough notes we took in May but am happy to have others rearrange, rewrite, etc.

[pradyunsg]

@brainwane Thank you so much for continuing to work on this! :D

We finally filed #6831 for tracking this.

I was thinking we can split this out into sub-pages that get fleshed out individually. For that, I made #6836 for adding the sub-section to add pages to. Having a sub-section allows us to have dedicated pages for both kinds of content easily:

• deep-dive into certain parts discussing edge cases and relevant standards/trade-offs (like say the downloading and index interaction logic)
• overview-style content (like the repo structure, or walkthrough an install).

[brainwane]

Thanks for the guidance and the new subsection. I rebased on that and turned what I'd written into some subpages for architecture/. I'll polish those a bit more and then mark this as ready for review and merging.

[pradyunsg]

Sounds great! Thanks @brainwane! :D

[brainwane]

I'm sorry but I do not have time to finish this. @cjerdonek is it possible for you to take over?

[pradyunsg]

If no one has time, I'm happy to add a WIP note to the contents of these documents, and merge this PR as is. We can come around to improving this at a later date, and something is better than nothing.

[brainwane]

I've added the WIP note to the document contents, so go ahead and ship it.

[pradyunsg]

Perfect! :)

[pradyunsg]

Thanks for all of your work on this @brainwane! ^>^