Add vendored template for Browser Performance Testing

What does this MR do?

Adds vendored template for Browser Performance Testing.

This mostly copies the example from the docs. There's a minor difference to support custom options. Given we now moved the job definition to a template, we need to provide users a simple way to set options. Without the SITESPEED_OPTIONS variable, they will have to redefine each script command just to redefine with options.

Examples in the documentation have also been updated to use the vendored template.

Solves #10528 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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