Add maximum model size spec to docs

[bmorcos]

Motivation and context:

Chris noticed this isn't listed anywhere... now it is!

Interactions with other PRs:

nengo-pynq!18
nengo-de1!14

How long should this take to review?

• Quick (less than 40 lines changed or changes are straightforward)

Where should a reviewer start?

Build the docs and take a look at the new section.

Types of changes:

• Non-code change (touches things like tests, documentation, build scripts)

Checklist:

• I have read the CONTRIBUTING.rst document.
• I have updated the documentation accordingly.
• I have included a changelog entry.
• N/A I have tested this with all supported devices.
• N/A I have added tests to cover my changes.
• N/A I have run the test suite locally and all tests passed.

Still to do:

These docs now reference pages that don't exist yet, so we must merge in the hardware docs first.

• Merge nengo-de1
• Merge nengo-pynq

[xchoo]

I think we should put the specific details into the respective board documentation.