Feature/5783: bring back 'rss.xml'

[myhro]

When trying to follow the Sourcegraph blog, I noticed that the RSS feed was missing. Digging through the issues, I realized that this was already reported on #5783.

This PR adds an rss.js script based on the upstream example, adapted to the repository blogposts/ directory structure. It also enables the RSS feed auto-build and auto-discovery.

@elzannewentzel @bretthayes can you please take a look?

Closes #5783.

[sourcegraph-bot]

Notifying subscribers in CODENOTIFY files for diff cec0b6646ac27af88c5de580fcc3ba6d832c9b5b...646685b9ffb5b8fc57cd3952a3669eaabc1e475e.

Notify	File(s)
@content-platform-team	.gitignore package.json scripts/rss.js src/pages/_document.tsx yarn.lock

[myhro]

Closing this as it didn't get traction in a month.