DOC: Added indexing views to roadmap
This adds the indexing copy vs. view proposal to the roadmap (#36195).

Given the length of the proposal, I've included a summary in the main `roadmap.rst`, with a link to the full proposal in a second document. Here's the summary:

The major change from the Google doc shared earlier is the recommendation for "Error on Write" over "Copy on Write". I think the proposal should recommend one over the other, and I think "error on write" is probably the way to go, but I'm fairly uncertain on this point.

cc @pandas-dev/pandas-core 