[WIP] Docs: improve parameter formatting using custom Sphinx domain

[eebasso]

Add a custom Sphinx domain for documenting parameters. Supports names containing characters like <, >, /, commas, etc.

This PR is inspired by the AMReX documentation of runtime parameters, which uses the Sphinx Python domain directive py:data in RuntimeParameters.rst. However, this fails for invalid Python variable names such as <species_name>.xmin,ymin,zmin that commonly occur in the WarpX documentation.

This domain solves that problem and allows use of parameter names such as <species_name>.xmin,ymin,zmin that do not work with the standard Sphinx Python domain.

The options of the fv:var directive build on the standard proposed by #6576

This is how the Simulation Time section rendered:

[RemiLehe]

Suggested change

	"T": ["Telsa", "Telsas"],
	"T": ["Tesla", "Teslas"],

[RemiLehe]

Btw, is this case sensitive? What happens if a user types "tesla"

[EZoni]

Just want to comment that a "user" in this context would be a contributor, correct? I think it's fair to request that contributors follow contributing guidelines (which in this case could include "write units in formulas when adding an input parameter to the documentation").

This is what I was trying to communicate during the meeting, as an alternative solution to over-engineering guardrails against variations of inputs, typos, etc. - alternative that relies on contributors following contributing guidelines and reviewers enforcing them.