Merge branch 'nav-issue-template' into 'master'

Add issue template for updating left sidebar

See merge request gitlab-org/gitlab!68490

.gitlab/issue_templates/Navigation - Left Sidebar Proposals.md

+<!-- This template is used for proposing changes to the left sidebar contextual navigation. This could include additions, removals, or general changes to overall hierarchy.-->
+
+### Proposal 
+
+<!-- Use this section to explain the proposed changes, including details around usage and business drivers. -->
+
+### Checklist
+
+- [ ] If your proposal includes changes to the top-level menu items within the left sidebar, engage the [Foundations Product Design Manager](https://about.gitlab.com/handbook/product/categories/#foundations-group) for approval. The Foundations DRI will work with UX partners in product design, research, and technical writing, as applicable.
+- [ ] Follow the [product development workflow](https://about.gitlab.com/handbook/product-development-flow/#validation-phase-2-problem-validation) validation process to ensure you are solving a well understood problem and that the proposed change is understandable and non-disruptive to users. Navigation-specific research is strongly encouraged.
+- [ ] Engage the [Editor](https://about.gitlab.com/handbook/engineering/development/dev/create-editor/) team to ensure your proposal is in alignment with holistic changes happening to the left side bar.
+- [ ] Consider whether you need to communicate the change somehow, or if you will have an interim period in the UI where your nav item will live in more than one place.
+- [ ] Once implemented, update this [navigation map in Mural](https://app.mural.co/t/gitlab2474/m/gitlab2474/1589571490215/261462d0beb3043979374623710d3f2d6cfec1cb) with your navigation change.
+
+/label ~UX ~"UI text" ~"documentation" ~"documentation" ~"Category:Navigation & Settings" ~"Category:Foundations" ~navigation