Rework Pulp installation instructions

[quba42]

Problem
The installation instructions are all over the place and (potential) users are confused. This applies to new users for sure, but just to be clear it also applies to people like myself with significant Pulp development and usage experience. I was recently looking around for how to put our production Pulp instance (installed using pulp_installer) on to solid foundations for future upgrades, and I am pretty much confused.

Another E.g. of a user who does not know where to start: https://discourse.pulpproject.org/t/pulp-installation-way-to-go/870

I am pleased to see, the most pressing of all installation doc issues already has a separate issue with a prio-list tag: #3834

However, this is not enough since there are many other places with outdated or confusing installation instructions.

Describe the solution you'd like

A concerted effort to re-work and unify all entry-points for installation instructions. Some concrete issues to start with:

• Remove references to pulp_installer as supported installation method #3834
• Entry-point explosion from pulpproject.org:
  • The website has a new user entrypoint (good!), but it also has all of the following:
    • https://pulpproject.org/pulp-ha/
    • https://pulpproject.org/pulp-in-one-container/
    • https://pulpproject.org/pypi-installation/
    • https://pulpproject.org/pulp-operator/
    • https://pulpproject.org/podman-compose/
  • Some of these include direct documentation, others include mainly links, some to the deprecated pulp_installer, others to youtube videos. Some link to each-other. There are links to the pulpcore GitRepo. There are links to the pulpcore installation docs. Most things are in various states of out of date, nothing is uniform.
  • Proposal: Since it is pretty much impossible to keep direct documentation on the website up to date, the website should mainly just point here: https://docs.pulpproject.org/pulpcore/installation/index.html The pulpcore docs should be the single entrypoint for up to date installation instructions. The main sources of truth for actual up to date installation instructions should probably be https://docs.pulpproject.org/pulp_oci_images/ and https://docs.pulpproject.org/pulp_operator/
• The pulpcore installation instructions should be opinionated about steering everyone to one (maximum two) main installation methods, most likely:
  • https://docs.pulpproject.org/pulp_oci_images/
  • https://docs.pulpproject.org/pulp_operator/
  • We are open source and everyone is free to do whatever they want (manual pypi installation, downstream packaging, etc.) but that does not mean we can't tell everyone who has no clear preference of their own to just go ahead and use the OCI images (or Operator if you've got a Kubernetes).

Describe alternatives you've considered
If we want people to use pulp, there is no alternative to up to date easy to find installation instructions!

Additional context
Just try the following little experiment:

1. Start here: https://pulpproject.org/
2. Try to find your way to step by step instructions for the default installation scenario for first time users who don't already have detailed infrastructure preferences.

[ggainey]

This is a fine idea. We're going to use this issue as an "epic", and link smaller targeted issues to it (like, say, one per-plugin for the plugin-installation-updates). Stay tuned!

To start with - see pulp/pulpproject.org#459 for current work that's related.