LLM Observability reorg, first phase

[cswatt]

What does this PR do? What is the motivation?

Moves around and tidies up page structure, with aliases, in preparation for further additions and sections.

Combines existing Python/Node.js pages into one SDK index page.

The setup flow (in particular, the "Setup" page) is pretty non-optimal right now, but this PR lays the groundwork for further improvements in the coming two weeks.

Merge instructions

Merge readiness:

• [x] Ready for merge

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Additional notes