Drop rss_sections from the user guide

[chalin]

While is see rss_sections in the docs, I don't see it referenced in the layouts. Am I missing something here?

I've done a global search through GitHub and only found one project's layout that uses this parameter: https://github.com/apache/beam/blob/50a3403a4742e1c9e264f57f4411969daeff4642/website/www/site/layouts/index.feed.xml#L23

[chalin]

If it isn't used, then we should strip it out of the docs and the config.

[chalin]

Ah, we lost that feature when we made the switch to using Hugo's template:

• Drop RSS layout in favor of Hugo's default #1948

I'd vote to clean it out of the docs as well. WDYT @emckean @LisaFC?

[LisaFC]

I can't see any reason to keep it in the docs given that it isn't currently used in the actual theme.

[LisaFC]

I can do the PR for removing it from the docs if you like.