1) Add a simplified `Default.md` issue description template for `www-gitlab-com`, 2) Document issue/MR default description template review policy, 3) add CODEOWNERS for default description templates

Why is this change being made?

The current default issue description template for www-gitlab-com is stored in the projects settings under Default description template for issues which is not under source code management and only a Maintainer or Owner can edit it. Similar to !104162 (merged) this MR will migrate the default issue description template from project settings into .gitlab/issue_templates/Default.md.

The existing issue description template was out of date, so I've updated it to be a list of suggestions for the issue filer. Once this MR is merged, I will need to clear out the Default description template for issues in the www-gitlab-com projects settings.

Existing default template in project settings

Please select a template from the top left dropdown before continuing.

This repository primarily relates to the logged out marketing website
https://about.gitlab.com/ 
and company handbook
https://about.gitlab.com/handbook/ 
This is not the correct repository for requests related to docs.gitlab.com, the GitLab product, or other items.

If you do not select an appropriate template then your issue might get lost. For more information about our workflows, please reference the handbook for the marketing website
https://about.gitlab.com/handbook/marketing/website/
or reference your team documentation.

### FAQ:

Q: Which template should I use?

A: Most people will use the "-website-bug-report" or "-handbook-update" issue templates. To request updates to existing pages or new content, please visit https://about.gitlab.com/handbook/marketing/growth-marketing/#requesting-support

There are also templates for writing various kinds of blog posts including the unfiltered category. Please see the blog handbook for instructions before you open an issue: https://about.gitlab.com/handbook/marketing/blog/#publishing-process 

The rest of the templates are likely specific to team, role, or special occasion.

Q: When will you get to this issue?

A: Your issue will be refined and then prioritized relative to other issues. Marketing's website development team is currently an impacted resource.

Q: How do I link to an issue template?

A: https://gitlab.com/gitlab-com/www-gitlab-com/issues/new?issuable_template=name-of-template

Q: How do I edit this FAQ? I have another question.

A: Submit an issue
https://gitlab.com/gitlab-com/www-gitlab-com/issues/new 
with FAQ in the title and assign it to @ brandon_lyon or another member of the website team
https://about.gitlab.com/handbook/marketing/website/

Additional Changes

• Document the description templates, their location, and review policy for the CoST Team to the CEO or CEO in the handbook
• Update CODEOWNERS for description template files to follow the review policy

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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