Review and adapt existing Kubeapps documentation to comply with new guidelines

[castelblanque]

Summary
Review and adapt existing Kubeapps documentation to comply with new guidelines defined in #5019.

Background and rationale
New guidelines for documentation have been added to Kubeapps in #5019 and PR #5020.
This newly required way of writing may differ from the existing documentation.

Description
This tasks consists in adapting/rewriting existing Kubeapps documentation to the new standard, for example:
from Under the hood, kapp-controller will create... in this document about Carvel packages, to Internally, ``kapp-controller`` will create....

Acceptance criteria
Documentation is rewritten meeting the new guidelines, where needed.

[ppbaena]

Summary:

• General documentation
• Tutorials
• How-to guides
  • Update How-to guides related to Forms after the revamping
  • Update How-to guide for Custom App View
  • Update how-to guide for multi-cluster support
• Background
  • Define architecture diagram of Kubeapps as code
• Reference
  • Update developer guides

[ppbaena]

Working on How-to guides:

• Access Control
• Dashboard
• Offline Installation
• Private Package Repository
• Using an OIDC provider with Pinniped

The rest of the How-to guides will be updated on specific issues as the team is currently working on issues related to them.

[ppbaena]

I've created specific issues for updating pending documentation to be updated and aligned to work in progress in different components (forms, multi-cluster, architecture, etc.).