[UX] Explore "Learn GitLab" item in group/project side nav

Follow up of #1632 (comment 469339780)

Deep dive on continuous onboarding items and the backend events we'll use from this issue to track if they're completed or not

Item order	Item copy	Link destination	Internal events used to determine if the item has been completed
1	Create / import a repository	Create or import your code into your Project (Repository)	First Git Write operation (gitlab-org/gitlab!48924 (merged)) or First repository imported or First repository mirrored
2	Setup Ci/CD	Set up CI/CD issue	First pipeline created (successful or not) (gitlab-org/gitlab!49010 (merged))
3	Submit a merge request	Submit a merge request	gitlab-org/gitlab!49002 (merged)
4	Invite your colleagues	Invite a colleague	First user added (gitlab-org/gitlab!49383 (merged))
5	Run a security scan	Run a security scan	Enabled/ran a security scan (Nicolas)
6	Submit an MR that auto-closes an issue	NOT IN THIS TEST	MR that auto-closes an open issue gitlab-org/gitlab!51903 (merged)
7	Start a free trial of GitLab Gold	Start a free trial of GitLab Gold - no credit card required 🚀	Trial started (gitlab-org/gitlab!49351 (merged))
8	Enable code owners	Set up code owners	Enabled code owners gitlab-org/gitlab!51925 (merged)
9	Enable require merge approvals	Set up required merge approvals	Enabled required merge approvals gitlab-org/gitlab!51987 (merged)

With items 8 and 9 within the list we should add an indicator to complete these you need to start a trial

Potential Solution

We'll add a learn GitLab entry in user's projects left side navigation. This page will reference all the steps necessary for new users to Set up their workspace, Plan and execute their work and deploy their app. To guide user to task completion, we'll start by using existing onboarding issues from the Learn GitLab project.

Flow

Variant 1-A

Variant 1-B

📐 Design specs