[DOCS] Link UHI+ docs, add shorthands to quickstart

[andrzejnovak]

• We've had reports of people struggling to find the UHI+ indexing, should feature more prominently in README and the docs.
• Also doc quickstart should include quickhand syntax which is less daunting and more commonly used

[henryiii]

AFACT, both README and Quickstart do use UHI+, and the QuickConstruct syntax (Quickstart does show both)?

[andrzejnovak]

Hmm, when I search README I only see this bit which doesn't imo look significant to new users.

hist's main indexing page https://hist.readthedocs.io/en/latest/user-guide/indexing.html should either link or copy info at https://uhi.readthedocs.io/en/latest/indexing.html#slicing

Also the super useful hist.tag.Slicer() functionality is missing here. Actually as far as I can tell this only lives in the docs in the API reference.

By "quickhand" I meant syntax like hist.new.Reg(10, 0, 1).Weight().fill(...) which doesn't seem to be what's at https://hist.readthedocs.io/en/latest/user-guide/quickstart.html