documentation: add llms.txt generation

[potatoboiler]

Issue #, if available: n/a

Description of changes: Add llms.txt to the root of readthedocs.io domain for LLM discoverability.

Testing done: manual READTHEDOCS_CANONICAL_URL='https://amazon-braket-sdk-python.readthedocs.io/en/stable/' tox -e docs produces the desired llms.txt file.

Merge Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your pull request.

General

• I have read the CONTRIBUTING doc
• I used the PR title format described in CONTRIBUTING
• I have updated any necessary documentation, including READMEs and API docs (if appropriate)

Tests

• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have checked that my tests are not configured for a specific region or account (if appropriate)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (e31289d) to head (123cf10).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #404   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           49        49           
  Lines         4967      4967           
  Branches       579       579           
=========================================
  Hits          4967      4967           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[potatoboiler]

https://amazon-braket-default-simulator-python--404.org.readthedocs.build/en/404/llms.txt