Stylistic choice: core/Core/The Core

[th]

intro/conceptual-overview.rst:

There's a few concepts I haven't introduced here: temporal properties, fairness, stutter-invariance, etc. All of these will be covered later. Hopefully, though, this is enough to give you a sense of what, if you decide to learn TLA+, you'll actually be able to do with it. If you're interested in continuing, check out the :doc:core </core/index> and setup.

core or Core or The Core (as in intro.rst)?

[hwayne]

I'm moving everything over to a |core| substitution so that it will be consistent.