docs: update docs about integration tests

What does this MR do?

This updates the docs about testing GitLab UI changes in GitLab. There are 2 different approaches being documented in different locations, one of them in a context that's specific to adding new component, even though integration tests might be relevant to other contexts.

By creating a dedicated section, those approaches will be more visible and it will make more sense to link to them when asking for integration MRs for changes that don't introduce new components.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.