docs: move chartmuseum info into private packages page

[HonkingGoose]

Changes:

• Move chartmuseum info into getting-started/private-packages.md
• Add link to private packages from helm-requirements/readme.md

EDIT: Updated the changes section, kept the original context at time of opening the issue.

Context:

@dudicoco said ¹ that the chartmuseum information that's placed in the Docker docs should go in the helm-requirements manager docs instead.

I don't know if I can use a h2 heading in this file, or if that will mess up the display of the published page. I think no other manager readme file has headings in it? The pattern to check would be lib/manager/manager-name/readme.md.
Turns out I'm not allowed to use h1 or h2 headings. 🙈

 FAIL lib/manager/metadata.spec.ts (262 MB heap size)
  ● manager/metadata › helm-requirements has readme with no h1 or h2

I'm not sure if this is a better place than in the Docker docs, it depends where people will think to look.

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please tick one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

Footnotes

1. https://github.com/renovatebot/renovate/issues/12093 ↩

[viceice]

🤔 i think this is better suited at helm datasource or our private modules page.

It should also have a hostType to not collide with default host rules.

[dudicoco]

@HonkingGoose this applies to standard helm repositories as well (not hosted via chartmuseum), so I believe the docs should reflect that

[HonkingGoose]

@HonkingGoose this applies to standard helm repositories as well (not hosted via chartmuseum), so I believe the docs should reflect that

That runs the risk of duplicating the information in multiple places though. I'd prefer to have a single location for this text. I'm okay with adding links to the canonical text in other places.

[renovate-release]

🎉 This PR is included in version 28.3.0 🎉

The release is available on:

• GitHub release
• 28.3.0

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