Fix additional build warnings (part 2)

[mmcky]

This PR fixes the remaining build warnings from the publish.yml workflow:

Fixes

1. getting_started.md:95 - Fixed {index} directive that was missing the required argument. Changed from multi-line format to individual {index} directives matching the English source format.

2. jax_intro.md:60 and numpy_vs_numba_vs_jax.md:62 - Created the missing lectures/_admonition/gpu.md file with the GPU admonition content from the English source repository.

Build Errors Fixed

/lectures/getting_started.md:95: ERROR: Directive 'index': 1 argument(s) required, 0 supplied
/lectures/jax_intro.md:60: CRITICAL: Directive "include": file not found: '_admonition/gpu.md'
/lectures/numpy_vs_numba_vs_jax.md:62: CRITICAL: Directive "include": file not found: '_admonition/gpu.md'

This is a follow-up to PR #56 which fixed the first set of build warnings.

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