new "Update Channels" post and docs updates around release process

A bunch of text changes bunched together to save us all time.

• blog post about https://gitlab.com/fdroid/update-channels
• added the official F-Droid PPA to the docs (ppa:fdroid/fdroidserver)
• updated Release Process doc with Weblate steps
• document PNG issues in Reproducible Builds

These all should be available on my staging site: https://eighthave.gitlab.io/fdroid-website/docs/