Plugin-E2E: Docs

[sunker]

Which areas does this feature request relate to

• [ ] Create Plugin
• [ ] Sign Plugin
• [X] Plugin E2E
• [ ] Documentation

Problem

Grafana Plugin E2E needs documentation.

Solution

Proposed structure:

├── introduction.md 
├── getting-started.md # installation, writing tests and setup basic CI workflow
├── locating-elements.md
├── feature-toggles.md
├── authentication.md
├── ci.md
├── setup-resources.md
├── how-to-test-data-source-plugins.md # frontend and backend
│   ├── configuration-editor.md
│   ├── query-editor.md
│   ├── panel-data-assertion.md
│   ├── custom-variable-editor.md
│   ├── template-variable-interpolation.md
│   ├── annotation-editor.md
├── how-to-test-panel-plugins.md
├── how-to-test-app-plugins.md
├── api ref # low priority, and only if we can auto-generate it from tsdoc
│   ├── assertions.md
│   ├── classes.md
│   └── fixtures.md

Alternatives

No response

Additional context

No response

Are you interested in contributing the solution?

• [X] Yes
• [ ] No

### Tasks
- [ ] https://github.com/grafana/plugin-tools/pull/701
- [ ] https://github.com/grafana/plugin-tools/pull/723
- [x] authentication-and-rbac
- [x] ci
- [x] provisioning
- [ ] assertions
- [x] how-to-test-data-source-plugins
- [x] how-to-test-panel-plugins
- [ ] how-to-test-app-plugins https://github.com/grafana/plugin-tools/pull/891
- [x] playwright-configuration
- [ ] best-practices
- [x] migrating-from-grafana-e2e
- [ ] api ref