Add link with additional CI minutes pricing

What does this MR do?

Per #213340 (closed), updating docs with an MVC to include a link to the customer portal with pricing for additional CI minutes so that users can find the pricing easier. The pricing is no longer explicit at https://about.gitlab.com/pricing/.

The docs page is linked to in a blog about CI minute changes for free users (in the "What if there's not enough minutes?" section) as well as in the FAQ section of the pricing page.

Screenshots

Customer portal has includes this pricing info

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team