chore: update sp-webpart sample to spfx v1.15

[gavinbarron]

Closes #1775

PR Type

• Refactoring (no functional changes, no api changes)
• Documentation content changes

Description of the changes

Updates the existing SharePoint webpart sample to use SPFx v1.15 Updates the documentation to make it clearer on how to load the webpart into the online workbench

PR checklist

• [x] Proje7ct builds (yarn build) and changes have been tested in at least two supported browsers (Edge + non-Chromium based browser)
• [ ] All public APIs (classes, methods, etc) have been documented following the jsdoc syntax
• [ ] Stories have been added and existing stories have been tested
• [ ] Added appropriate documentation. Docs PR:
• [ ] License header has been added to all new source files (yarn setLicense)
• [x] Contains NO breaking changes

Other information

Sample built and tested on both Node v14 and v16

[ghost]

Thank you for creating a Pull Request @gavinbarron.

This is a checklist for the PR reviewer(s) to complete before approving and merging this PR:

• [ ] I have verified a documentation PR has been linked and is approved (or not applicable)
• [ ] I have ran this PR locally and have tested the fix/feature
• [ ] I have verified that stories have been added to storybook (or not applicable)
• [ ] I have tested existing stories in storybook to verify no regression has occured
• [ ] I have tested the solution in at least two browsers (Edge + 1 non-Chromium based browser)