Remove store singleton pattern from docs

What does this MR do?

Removes from the documentation the pattern of exporting a default Vuex store singleton, favouring only a factory function.

This pattern is a bad practice largely because it can lead to stale state preserved between tests, or implicit (rather than the preferable explicit) coupling between different Vue applications on the same page that use the store.

This came up in an MR review: !34742 (comment 379821497).

cc @justin_ho

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