Add Intro section to Code Suggestions Tab

What does this MR do and why?

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/422629. With this MR we are introducing an empty state for the Code Suggestions tab, for when there is no purchased Add-on. Other states will be introduced with follow-up work. Note that the empty state is missing an illustration. It will be added with: https://gitlab.com/gitlab-org/gitlab/-/issues/423350

This MR also refactors a bit of code to make the location of some files consistent with the new tab approach.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Condition	Expectation	Screenshot
In SM Mode
	No tab should appear	Screenshot
In SaaS mode
-- With Feature flag disabled for group
	No tab should appear	Screenshot
-- With Feature flag enabled for group
---- With no purchased add-on data
	Shows empty state	Screenshot
---- With purchased add-on data
	Shows nothing	Screenshot
---- With an error in the add-on data request
	Shows empty state	Screenshot

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

In your rails console:

1. Customers Dot needs to be setup in order to view the billing page for a group

Feature.enable(:enable_hamilton_in_usage_quotas_ui, Namespace.find(your-group-id)

Group with subscription

1. Create a Group
2. Buy a Subscription for your group from Settings > Billing

Purchase Add-on

Run the following in Rails console:

1. Find the namespace

namespace = Namespace.find(your-group-id)

2. Create an Addon

add_on = GitlabSubscriptions::AddOn.find_or_create_by!(name: "code_suggestions") {|e| e.description = "Test"}

3. Create an add-on purchase

GitlabSubscriptions::AddOnPurchase.create!(add_on: add_on, namespace: namespace, expires_on: 1.month.from_now, quantity: 20, purchase_xid: 'A-S0001')

SaaS mode

1. Start GDK in SaaS mode GITLAB_SIMULATE_SAAS=1 gdk start

SM mode

• Start GDK in SM mode GITLAB_SIMULATE_SAAS=0 gdk start

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.