[docs] More updates for docs-assembler

[colleenmcginnis]

Adds a reference/toc.yml file, move individual reference toc entries from docset.yml to the new reference/toc.yml file. This also moves the intro content from docs-content to this repo so the intro can be at elastic.co/docs/reference/elasticsearch/.

[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)

[leemthompo]

🦭

[Mpdreamz]

I did spot the root name is correct now:

And the breaking changes sub pages are named incorrectly.

Unrelated to this PR though

[Mpdreamz]

Needs an index.yml file.

[colleenmcginnis]

I'm confused why I'm getting an error now... isn't this the pattern we used in https://github.com/elastic/docs-content/pull/897/files#diff-638bebef2d25c4bc8b59b234b825533f78bad25e4422581025d60f78a4ccc770? I must be missing something else.

[Mpdreamz]

@colleenmcginnis we still need toc_max_depth in the config to enable multiple levels of toc definitions.

https://github.com/elastic/docs-content/blob/main/docset.yml#L2

[colleenmcginnis]

Is this not the right spot for it? https://github.com/elastic/elasticsearch/pull/125509/files#diff-165b79089c3088414476471a320e02688a45f8a6d2d94fbc784ceae8ad5cae04R2

[Mpdreamz]

Ahh this check in docs-builder is overly restrictive.

It only allows a mix of file and toc in the docs-builder and docs-content repositories:

https://github.com/elastic/docs-builder/blob/7785afd41a6c7bc54f928c586b127a6a16b02378/src/Elastic.Markdown/IO/Configuration/TableOfContentsConfiguration.cs#L85

Will add a fix.

[Mpdreamz]

That fixed it :)