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 
hmm ci/checks are failing General question about error / copy on write (forgive me if this has been answered elsewhere), but does the same hold when we have a parent / view but modify the parent instead of the view? For example, supposing there's error on write and we have `s = df["a"]`; does setting on `s` raise but changes to `df["a"]` would still silently propagate? > General question about error / copy on write (forgive me if this has been answered elsewhere), but does the same hold when we have a parent / view but modify the parent instead of the view? For example, supposing there's error on write and we have `s = df["a"]`; does setting on `s` raise but changes to `df["a"]` would still silently propagate?

I'm not sure that we have discussed that???  Today the propagation is both ways, when using `df["a"]`, but doesn't happen with `df[["A"]]`.  I think the proposal is that for a view that is a `Series`, it remains both ways, but for a view that is a `DataFrame`, modifying the underlying `DataFrame` would silently propagate to the view. Let's try to keep the discussion in the issue. This PR was opened prematurely.

The "modifying the parent" issue is mentioned in the proposal at https://github.com/pandas-dev/pandas/pull/36988/files#diff-8a818e230eb272faebe3cf4b7b164a29e0311a42a09a8bd79d17d43d3249f4d3R231.