Update README with Documentation roadmap

[imath]

@emaralive @vapvarun @dcavins @renatonascalves

As discussed during September 4 docs chat, here's the PR we can improve to explain our goals/roadmap about BuddyPress documentation.

[emaralive]

I think we should archive the current master as a branch, prior to merging this pull request, this allows for an easy method to see/retain the 1st (original) version, if you will.

With this pull request we can then:

• Remove the data folder & it's content, since we would have an archive branch that has this info, in the event we need to reference anything from that content.
• Remove 1-migrate-task.yml from the .github/ISSUE_TEMPLATE folder since it is now obsolete and will exist in the archive branch.
• Remove 3-ignore-task.yml from the .github/ISSUE_TEMPLATE folder since it is now obsolete and will exist in the archive branch.
• Rename 2-add-task.yml to 1-add-task.yml, keeping a similar numeric prefix.
• Rename 4-update-task.yml to 2-update-task.yml, keeping a similar numeric prefix.
• Create a folder named roadmap, which will contain any detailed info that further describes/defines the top-level roadmap (as spelled out by the final version of the README.md), e.g., there would be links from the roadmap (README.md to whatever detailed .md files that are created).

Or, someone else can come up with a better idea and we can do that or stick with the PR as originally submitted.