[docs] GKE TPU guide: Fix var, add TPU types per legion link

[spencer-p]

Description

• The guide sets up a COMPUTE_ZONE environment variable which is later referenced as $ZONE. Changed to use ZONE consistently.
• There is a note about the TPU v4 location -- added a link to TPU regions and zones next to it for users trying other types.

Related issues

none

Additional information

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[gemini-code-assist]

Code Review

This pull request updates the GKE TPU cluster documentation by renaming the ZONE environment variable placeholder and adding a reference for other TPU types. The reviewer suggested improving consistency by using 'ZONE' as the placeholder value and using a standard URL format for the TPU locations link.

[ryanaoleary]

LGTM after the two gemini comments are resolved

[github-actions]

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[ryanaoleary]

not stale, cc: @edoakes needs core team review