add nwb assistant to home page of docs

[magland]

Motivation

At the recent NWB Docathon I created this AI assistant for NWB. This PR adds a button "Open Assistant" to the home page of the rendered docs, as shown:

When the user clicks the button, it opens NWB Assistant in an iframe, as shown:

For the user, the basic (cheaper) models are available for free. If they want to use the more advanced/expensive models, they need to put in their own openrouter key. There are instructions in the assistant app.

How to test the behavior?

Build the docs using cd docs && make html. Then host locally via python3 -m http.server 8000 --directory _build/html.

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running ruff check . && codespell from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.83%. Comparing base (f52d327) to head (584f6c5).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #2076   +/-   ##
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  Coverage   94.83%   94.83%           
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  Files          28       28           
  Lines        2750     2750           
  Branches      708      708           
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  Hits         2608     2608           
  Misses         82       82           
  Partials       60       60           

Flag	Coverage Δ
integration	74.18% <ø> (ø)
unit	84.83% <ø> (ø)

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