Eliminate local copies of .xml feeds on the OCaml Planet

[sabine]

Resolves #1242.

• All .xml feeds of the OCaml Planet are removed from the repository. ood-gen now has a planet_feed option to generate the OCaml Planet's feed.xml at build time.
• https://ocaml.org/feed.xml serves a feed with all posts of the last 90 days - now also includes the ones hosted on ocaml.org as well as the individual external links. Before, only the ones coming from sources in planet-sources.yml were part of the feed.
• Removed some dead sources from planet-sources.yml
• Individual external links, as well as posts hosted on ocaml.org can now also specify a source for the RSS feed of the OCaml Planet.
• We now list the name of the source as author, not the id.
• updated time of the global feed.xml is now the updated time of the most recent post
• individual external links as well as posts hosted on ocaml.org must now have a RFC3339 date, since they're part of the global feed.
• Better error messages: decode reports in which file something is wrong
• Source, Feed, Scraper are now modules within planet.ml, for better code readability

Note: Right after merging this, the scraper branch needs to be rebased on main, or else the scraper will do weird things.

ETA: I only split this up in two commits because it's impossible to review the code on the GitHub UI with the removal of .xml feeds and regenerating of posts. The commits do not work individually, only together. The first commit is the one that changes the logic.