Update doc/development/experiment_guide/experiment_code_reviews.md, doc/development/experiment_guide/index.md, doc/development/experiment_guide/implementing_experiments.md, doc/development/experiment_guide/experiment_rollout.md, doc/development/experiment_guide/testing_experiments.md

We’ve removed the concept of using `window.gon` in the client, so we can
now remove the publishing overrides that were keeping that logic. We
also remove the `#publish_to_client` method to encourage calling the
`#publish` method directly.

- deprecated and shouldn't be referenced for local development.

Changelog: removed

- Deprecates the usage of `publish_to_database` as well

Rename experiment_subjects group_id column to namespace_id
to make it easier to store and retrieve namespaces.

Changelog: changed

Use our experiment_subjects table for recording
experiment subjects build with GLEX.

Changelog: added

Vue recommends to have multi-worded components to not conflict them with
HTML elements or web components.

- Reuse the same example experiment from earlier in the docs
- Be consistent in use of "multiple variants" rather than "multivariate"
- Clarify a few statements

This commit moves a subheading from a gerund to present tense, adds
hard line wraps, and brings the new work closer to GitLab tone and
style.

This adds a Vue component that can be used for experiments that are
conducted with `gitlab-experiment`.

To minimize merge conflicts, I'm capturing the line wraps in a
separate commit, and will start tackling my edits in the next commit.