docs(box): add git config params and endpoint docs

[CahidArda]

Summary

• Document optional git identity fields on box creation: git_user_name, git_user_email
• Document defaults: Upstash Box / box@upstash.com
• Document new endpoint: PUT /v2/box/:id/git-config
• Document partial update rule (at least one field required)
• Document response shape and immediate runtime application behavior
• Document commit author overrides for POST /v2/box/:id/git/commit: author_name, author_email

Notes

• This PR is generated from BOX-90 API updates.
• Endpoint paths and examples were updated to match current API behavior.

[linear]

BOX-90 git config