3.21 release blog post

Created by: sqs

The work remaining is to trim down the list of changelog entries to only include the most important things; and add or change the URLs and categories for each section if they are incorrect. For example, see how the 3.20 and 3.19 posts at https://about.sourcegraph.com/blog/ are a subset of the changelog and many of the rows link to the doc pages, not the GitHub PR.

If you change an entry's wording here, you should also change it in the CHANGELOG.md file in the sourcegraph/sourcegraph repo. You can do this manually, or you can use the script I wrote to regenerate the whole list for the blog post YAML from the CHANGELOG.md file:

# in the root dir of this repo, make sure the latest sourcegraph/sourcegraph repo
# CHANGELOG.md file is at ../sourcegraph/CHANGELOG.md, then run:
go run ./bin/generate_changelog_items.go -versions 3.21 -i ../sourcegraph/CHANGELOG.md