Docs: add bulk update section to per-user credit caps documentation

What does this MR do and why?

Adds a #### Bulk update overrides subsection under #### Set per-user cap overrides in the GitLab Credits dashboard docs, covering the CSV paste workflow shipped in GitLab 19.5.

To access the feature, users select Bulk update under Per-user cap overrides on the Credit caps page. A modal appears that accepts a CSV list.

The section documents:

• Steps to open the bulk update modal and paste the CSV list
• CSV column format and accepted values
• Note that header rows are not accepted
• Limits and error behaviour (atomic batch, unresolved usernames fail the whole request)

Implementation status

Part of https://gitlab.com/gitlab-org/gitlab/-/work_items/630981

Piece	Work item	MR	Status
Feature flag credit_caps_bulk_update + scaffold button	https://gitlab.com/gitlab-org/gitlab/-/work_items/630982	!257651 (merged)	✅ Merged
BE: username on BudgetCapUserOverrideInput	https://gitlab.com/gitlab-org/gitlab/-/work_items/630983	!258407 (merged)	✅ Merged
FE: bulk update overrides modal	https://gitlab.com/gitlab-org/gitlab/-/work_items/630982	!257948 (merged)	✅ Merged
FE: CSV parser	https://gitlab.com/gitlab-org/gitlab/-/work_items/630984	!258244 (merged)	✅ Merged
FE: integrate modal with BE	https://gitlab.com/gitlab-org/gitlab/-/work_items/630985	!258465 (merged)	✅ Merged
FE: CSV parser improvements (follow-up)	https://gitlab.com/gitlab-org/gitlab/-/work_items/631895	!259381 (merged)	✅ Merged
Documentation update	https://gitlab.com/gitlab-org/gitlab/-/work_items/632125	!259654 (merged)	⬅️ Current
Feature flag rollout	https://gitlab.com/gitlab-org/gitlab/-/work_items/631018

Closes https://gitlab.com/gitlab-org/gitlab/-/work_items/632125