maybe redirect readme era docs/ to stable docs

[cburroughs]

Currently (as of #35) the old readme era urls point to 2.18 docs, which were the live version at the time of the cutover. So for example https://pantsbuild.org/docs/rules-api-concepts/ redirects to https://www.pantsbuild.org/2.18/docs/writing-plugins/the-rules-api/concepts with a "This is documentation for Pantsbuild 2.18 (deprecated), which is no longer actively maintained." banner.

Since #256 we now have semi stable docs urls, not just versioned docs. So if one clicks through the above banner one eventually taken to https://www.pantsbuild.org/stable/docs/writing-plugins/the-rules-api/concepts

Maybe we should cutout the middle and redirect https://pantsbuild.org/docs/rules-api-concepts/ to https://www.pantsbuild.org/stable/docs/writing-plugins/the-rules-api/concepts ?

Pro: This is almost certainly what people want in most cases.
Con: Eventual linkrot as doc urls change.