Load experiments via tracking options on runtime

What does this MR do?

Related to #328008 (closed)

There was no clear indication for providing tracking options on runtime on Vue, this MRs tries to make it clear in our docs.

It also cleans up the original use case for the related issue:

created() {
  // `this.experiment` was provided at runtime
  const trackingMixin = Tracking.mixin({ experiment: this.experiment });
  // To mimic the "update" of the mixin logic, a new instance was created
  const trackingInstance = new Vue({
    ...trackingMixin,
    render() {
      return null;
    },
  });
  this.track = trackingInstance.track;
},

The case above could have been solved by:

computed: {
  tracking() {
    return { experiment: this.experiment },
  },
},

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.