Update docs with `confval` definitions

[fmigneault]

Description

Currently, configuration settings are defined using envvar definitions with custom structures for default/types. For example:

Magpie/docs/configuration.rst

Lines 549 to 554 in 2e57bef

	.. envvar:: MAGPIE_PORT
	[:class:`int`]
	(Default: ``2001``)
	Port URL part of `Magpie` application to rebuild the full :envvar:`MAGPIE_URL`.

Sphinx provides a way to define confval directives, which can be referenced by :confval:`<ref>` (rather than :envvar:`<ref>`).

Using confval allows to directly making use of the directive to define :type: and :default::

.. confval:: the_answer
   :type: ``int`` (a *number*)
   :default: **42**

   This is a setting that controls the value of the answer.

Which would be rendered as:

---

the_answer¶

Type:

int (a number)

Default:

42

This is a setting that controls the value of the answer.

---

References

• https://www.sphinx-doc.org/en/master/usage/domains/standard.html#directive-confval