blogpost: Introducing a new database for code intel

Created by: efritz

Closes https://github.com/sourcegraph/sourcegraph/issues/14009.

This is a quick blog post to coincide with the release of 3.21 and the introduction of a second database instance, which may throw some site admins off at first. This blog post is meant to explain the reasons we chose to add a second database and how it might affect existing users.

I have also written the story detailing the weird code intelligence outage that happened in the cloud instance when applying this change, but am planning on publish it on my personal blog instead. I think the story and circumstances are interesting (and should be made available), but is not necessarily something we want to immortalize directly next to the announcement for the change that caused it. See https://github.com/efritz/blog/pull/13 for the outage story and feel free to leave comments on that PR as well.

My thinking may be skewed here, so I'm happy to hear alternative viewpoints!