Add docs for config system and presets

[tbekolay]

Description:
This adds a new page to the user docs explaining the config system and the presets (currently just the ThresholdingEnsembles preset). Note that this tries not to overlap too much with the config.ipynb notebook, but links to it instead.

Motivation and context:
We discussed at the last dev meeting that these should be added to the docs before the 2.2.0 release.

How has this been tested?
I've built the docs with these changes. They build without warnings and look pretty deece.

Where should a reviewer start?
Files changed tab.

How long should this take to review?

• Average (neither quick nor lengthy)

Types of changes:

• Bug fix (non-breaking change which fixes an issue) -- sort of

Checklist:

• I have read the CONTRIBUTING.rst document.
• I have updated the documentation accordingly. 😉
• I have included a changelog entry.
• I have added tests to cover my changes. (nothing to test)
• All new and existing tests passed.

[Seanny123]

LGTM.

[jgosmann]

I found one sentence that I had trouble with understanding. I rewrote it and made it a bit more specific regarding the usage of object vs. class. I also moved the config documentation behind the network documentation because that seems to me like a more logical progression (especially because the config documentation mentions networks in a few places).

One thing that slightly irritates me is that all the headings in the same level of the navigation are nouns (e.g. "Networks"), where this one title is a whole phrase ("Setting parameters with Configs"). But shortening it to "Configs" or "Configurations" makes it less expressive.

So: LGTM 🍰

[tbekolay]

Reword looks good, but I'd like to revert the move from before network docs to after. The network object itself is documented in frontend_api.rst, which comes first; the networks.rst doc only documents the specific networks that are included with Nengo. In terms of abstraction levels, I think presets are lower level than networks because networks use presets, and not the other way around, so I would put them first.

One thing that slightly irritates me is that all the headings in the same level of the navigation are nouns (e.g. "Networks"), where this one title is a whole phrase ("Setting parameters with Configs").

This is a good point... I've been trying to think of a noun version of that title and nothing is popping out at me. Could switch it to just be "The config system" and rearrange the headings a bit... alternatively, can change the headings of the other sections to be more free-form. I think probably this is good enough for now, can revisit later.

[tbekolay]

Pushed commits to revert and fix a line that was too long; if you're okay with it this way @jgosmann please squash and merge! 👍 🌈 👍

[jgosmann]

The network object itself is documented in frontend_api.rst, which comes first; the networks.rst doc only documents the specific networks that are included with Nengo.

Right!