New_cdot_navigation docs integration

What does this MR do and why?

Related to customers-gitlab-com#7008 (closed)

Cannot be merged until customers-gitlab-com#6372 (closed) is complete (new_cdot_navigation feature flag rollout).

An integration branch of updates for the new_cdot_navigation feature.

Updates:

Documentation link	Original MR	Status
payment methods documentation	Update CustomersPortal payment docs (!136124 - merged)	Reviewed
account owner/company details documentation, GL account linking documentation	Update personal and company information docs (!136126 - merged)	Reviewed
GitLab SaaS/SM & QSR payment method pages	Update GitLab SaaS/SM payment docs (!136520 - merged)	Reviewed
Billing/subscription contact (New section)	Add documentation for billing contacts (!136525 - merged)	Reviewed
Contact troubleshooting (New section)	Billing contact troubleshooting (!137401 - merged)	Reviewed

to support the new CDot Navigation UI flow

Screenshots or screen recordings

See original MRs for examples of new UI flows

How to set up and validate locally

• Run the review-docs-deploy pipeline job
• Click on the deployed app to view changes

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.