Add docs for helm_extension mechanism

[viniciusdc]

Reference Issues or PRs

What does this implement/fix?

Includes doc section for helm_extension mechanism that was added as part of nebari-dev/nebari#865

Put a x in the boxes that apply

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features not to work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Testing

• Did you test the pull request locally?
• Did you add new tests?

Documentation

Access-centered content checklist

Text styling

• The content is written with plain language (where relevant).
• If there are headers, they use the proper header tags (with only one level-one header: H1 or # in markdown).
• All links describe where they link to (for example, check the Nebari website).
• This content adheres to the Nebari style guides.

Non-text content

• All content is represented as text (for example, images need alt text, and videos need captions or descriptive transcripts).
• If there are emojis, there are not more than three in a row.
• Don't use flashing GIFs or videos.
• If the content were to be read as plain text, it still makes sense, and no information is missing.

Any other comments?

**Maybe this is already in the docs, and I just didn't find it; if that's the case, feel free to close it.

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[marcelovilla]

LGTM! Thanks @viniciusdc!