docs: Helm Migration guide could have a mini-toc at the top

[dholbach]

The TOC on the right hand side of https://toolkit.fluxcd.io/guides/helm-operator-migration/ is very crowded, let's add a mini-TOC at the top of the document to provide links to:

• discussion of breaking changes
• api spec changes
• migration strategy
• FAQ

[ChrisJBurns]

@dholbach Is the mini-toc going to be an addition to the right-hand side TOC? Additionally, when you say "discussion of breaking changes", do you mean the "Overview of changes"?

[dholbach]

My thinking was to have it just at the top of the doc itself. And yes - find a way to convey that "overview of changes" also includes breaking changes to be aware of.

[kingdonb]

I could add a table of links, but the docs have gone through some formatting changes since this was posted.

If you think we should use some formatting like the index page that is found at https://fluxcd.io/docs/guides/ then that's no problem, I can take this on. I think since the migration guide isn't changing too frequently, we can just curate the mini-TOC by hand, and it's OK if it doesn't refer to every heading but only the most important ones. WDYT?

(Assigning to myself)

[dholbach]

Not sure how much of an issue this still is?