Allow :sections[1:] etc. in permalinks config

[bep]

I'm in the process of fixing the rather odd section setup in the Hugo Docs site. By putting the documentation below a documentation page we will, by default, get URLs on the form:

https://gohugo.io/documentation/content-management/urls/#permalinks

Vs the old:

https://gohugo.io/content-management/urls/#permalinks

Which in itself would no be too bad, but it would break a lots of links. We could create redirects for them, but I'd rather avoid that. But I do think it would be useful for others to have a way to only include parts of the sections in your URL, so I suggest we expand the permalinks syntax a little to support the variants below:

:sections[1:] // all but the first
:sections[0] // the first
:sections[:2] // first two
:sections[last]

And variations of the above. We need to do some bounds checking and return an empty string when out of bounds.

[github-actions]

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