Add license and purchase link to getting started

[bmorcos]

Motivation and context:

There was no mention of a license or any links from which to by one, now there is.

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

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:

[xchoo]

I'm wondering if we should change the "NengoFPGA License" page heading to something else to avoid confusion between that and the NengoFPGA board licenses?

[xchoo]

Or move it to a "Project information" group like in Nengo-DL?

[bmorcos]

I changed the software license heading... Unsure if we want to create a project section or not :/

[bmorcos]

I Made a new section like NengoDL, looks good!