Update documentation and support pages for inviting billing account managers

Resources

1. Design: Figma
2. Implementation plan: Blueprint
3. Main epic: Enable customers to self-service manage multipl... (&10495)

Implementation

• Update customer docs (add a billing account manager section) with new functionality. Mention that the invite is valid for a period of 7 days. MR: gitlab!192304 (merged)
• Update Support Workflow for associating purchases. MR: gitlab-com/content-sites/handbook!13889 (merged)
• Update https://support.gitlab.com/hc/en-us/articles/17767356437148-How-to-transfer-subscription-ownership. Thread: #6650 (comment 2523353366)
• We should make sure to add description of when the customer accepts the invite, but is logged into gitlab.com with a different email, e.g. when the public email is sent the invite and they have a different email to log into the account with. See the conversation here for more information: https://docs.google.com/document/d/1vYqRzgGfnNesNnC-ksge4DsH6pliYzvjblL5xOpgA5o. See this discussion thread: #6650 (comment 2559744147)