docs: add api key claim instructions

[meirk-brd]

Description

The authentication section now includes a direct signup link and clearer instructions for obtaining and using the API key.

Fixes # (N/A - documentation improvement)

New Package?

Did I fill in the tool.llamahub section in the pyproject.toml and provide a detailed README.md for my new integration or package?

• Yes
• No (this is an update to an existing package)

Version Bump?

Did I bump the version in the pyproject.toml file of the package I am updating? (Except for the llama-index-core package)

• Yes
• No (documentation-only change, no version bump needed)

Type of Change

Please delete options that are not relevant.

• Documentation update (improved authentication instructions with UTM tracking)

How Has This Been Tested?

Your pull-request will likely not be merged unless it is covered by some form of impactful unit testing.

• I added new unit tests to cover this change
• I believe this change is already covered by existing unit tests (documentation-only change, no code changes)

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas (N/A - documentation only)
• I have made corresponding changes to the documentation
• I have added Google Colab support for the newly added notebooks (N/A - no new notebooks)
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works (N/A - documentation only)
• New and existing unit tests pass locally with my changes
• I ran uv run make format; uv run make lint to appease the lint gods (documentation-only change)