Properly support global variables in API docs (.. py:data::)

[Eric-Arellano]

This builds on top of #935, which is about broken cross-references. This issue is to also have a more intentional visual design for .. py:data::, such as an MDX component like <GlobalVariable> (name tbd).

unannotated screenshot

annotated screenshot

offending code

documentation/docs/api/qiskit/dev/circuit.mdx

Lines 883 to 893 in 394ca77

	Qiskit ships with a large set of predefined equivalence relationships for all of its standard gates. This base library is called [`StandardEquivalenceLibrary`](#qiskit.circuit.StandardEquivalenceLibrary "qiskit.circuit.StandardEquivalenceLibrary"), and should be treated as immutable.
	**qiskit.circuit.StandardEquivalenceLibrary**
	A [`EquivalenceLibrary`](qiskit.circuit.EquivalenceLibrary "qiskit.circuit.EquivalenceLibrary") that stores of all Qiskit’s built-in standard gate relationships. You should not mutate this, but instead either create your own [`EquivalenceLibrary`](qiskit.circuit.EquivalenceLibrary "qiskit.circuit.EquivalenceLibrary") using this one as its `base`, or modify the global-state [`SessionEquivalenceLibrary`](#qiskit.circuit.SessionEquivalenceLibrary "qiskit.circuit.SessionEquivalenceLibrary").
	Qiskit also defines a shared global-state object, [`SessionEquivalenceLibrary`](#qiskit.circuit.SessionEquivalenceLibrary "qiskit.circuit.SessionEquivalenceLibrary"), which is the default equivalences used by various places in Qiskit, most notably the [`BasisTranslator`](qiskit.transpiler.passes.BasisTranslator "qiskit.transpiler.passes.BasisTranslator") transpiler pass. You should feel free to add your own equivalences to this using its [`add_equivalence()`](qiskit.circuit.EquivalenceLibrary#add_equivalence "qiskit.circuit.EquivalenceLibrary.add_equivalence") method, and they will be automatically picked up by default instances of the [`BasisTranslator`](qiskit.transpiler.passes.BasisTranslator "qiskit.transpiler.passes.BasisTranslator").
	**qiskit.circuit.SessionEquivalenceLibrary**
	The default instance of [`EquivalenceLibrary`](qiskit.circuit.EquivalenceLibrary "qiskit.circuit.EquivalenceLibrary"), which will be used by most Qiskit objects if no library is manually specified. You can feel free to add equivalences to this using [`add_equivalence()`](qiskit.circuit.EquivalenceLibrary#add_equivalence "qiskit.circuit.EquivalenceLibrary.add_equivalence"). It inherits all the built-in rules of [`StandardEquivalenceLibrary`](#qiskit.circuit.StandardEquivalenceLibrary "qiskit.circuit.StandardEquivalenceLibrary").

I propose the design should look the same as our other components: a purple or blue left bar, header inside the component (#1395) with whatever the correct surrounding level would be, and ensure there is a <span id> or that the header has the ID.

[jakelishman]

Oh boy - "A EquivalenceLibrary" haha. No points for English for me.