Resolve "Move Templates Category from import"

What does this MR do?

Dissolves the templates category (which was formally only focused on project templates) and integrates templates into the respective groups

• projects templates-> groupsource code
• Ci yaml templates-> grouppipeline authoring
• pages templates-> grouprelease

closes gitlab-org/gitlab#332212 (closed)

Action items after this MR is approved:

@sarahwaldner:

• Capture relevant info from direction page and put it on a parent epic for Project Templates - Epic

@hdelalic:

• Make sure that all remaining issues in gitlab-org&2767 are in the %Backlog
• Update the label ~Category:Templates to ~Category:Source Code for all issues in gitlab-org&2767
• Add the devopscreate and groupsource code labels to all issues in gitlab-org&2767
• Add the devopscreate and groupsource code labels to all epics in gitlab-org&2767
• Work with the Quality team to REMOVE ~Category:Templates label from the Triage Bot automation gitlab-org/quality/triage-ops#712 (closed)
• Delete the Category:Templates entry in categories.yml
• Delete all features categorized under ~Category:Templates in features.yml
• Remove all mentions of Templates as a CATEGORY in the handbook
• Remove all mentions of Templates as a CATEGORY from the website
• Remove all mentions of Templates as a CATEGORY from the docs
• Delete the direction page for ~Category:Templates and all associated files

Approvals

Merge requests with changes to stages and groups and significant changes to categories need to be created, approved, and/or merged by each of the below:

Merge requests with changes to stages and groups and significant changes to categories need to be created, approved, and/or merged by each of the below:

• EVP of Product
• VP of Product Management
• The Product Director relevant to the stage group(s)
• The Engineering Director relevant to the stage group(s)
• CEO

The following people need to be on the merge request so they stay informed:

• Chief Technology Officer
• VP of Development
• Director of Quality Engineering
• Engineering Productivity (by @ mentioning @gl-quality/eng-prod)
• The Product Marketing Manager relevant to the stage group(s)

Screenshots (strongly suggested)

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.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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