Implement new snippet plugin including offset functionality

[snneji]

Context

To prevent duplication in docs we want to implement a snippets feature that meets our needs. Do this after #3769 is closed.

Tasks

1. Implement the snippets plugin from the result of [SPIKE] Which content re-use plugin should we use?  #3769.
2. Implement the offset functionality for the new plugin so that snippets can be included at the correct heading level in different places. For example, this one.

[csantanapr]

I asked in material-mkdocs about it squidfunk/mkdocs-material#3581

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[abrennan89]

One of the nice things about asciidoc > markdown+mkdocs is that it provides this out of the box by using leveloffsets with inclusions: https://docs.asciidoctor.org/asciidoc/latest/directives/include/ (no add ons required)

I would still like to keep this on the table for someday future discussion, since I think it still solves a lot more of our wants / needs than markdown.

[abrennan89]

Doesn't seem to be anything we can do about this one for now, closing until we find something else we can use, either for mkdocs or move to asciidoc etc.