fix gcp storage bucket binding docs

[MyMirelHub]

Description

Fixes multiple issues in the GCP Storage Bucket binding documentation (#5156):

Payload corrections:

• Fixed move, copy, and rename operation payloads to correctly place destinationBucket/newName in data instead of metadata
• Split parameter documentation into "metadata parameters" and "data parameters" sections

New documentation:

• Added comprehensive sign operation documentation with request/response examples
• Added signTTL to the spec metadata fields table
• Added note on backward-compatible name metadata key

Fixes:

• Removed duplicate "GCP Credentials" section
• Fixed trailing commas in bulkGet example and create response JSON
• Corrected bulkGet curl examples to use bulkGet instead of bulkget
• Fixed typos: "kobjects" → "objects", "retuned" → "returned"
• Fixed anchor links to match section headings
• Added sign operation to binding support operations list

[nelson-parente]

Pre-review for @marcduiker handoff: LGTM. Comprehensive fix for the GCP Storage Bucket binding docs: corrects move/copy/rename payloads (destinationBucket/newName now correctly in data), adds the sign operation with full request/response example, removes the duplicate GCP Credentials section, fixes the bulkget → bulkGet case bug in curl examples, and cleans up several typos. The backward-compat note for the name metadata key is a good touch. Safe to merge after CI.