Document Web IDE Settings Sync functionalities

Issue:

• Tech doc: gitlab-web-ide-vscode-fork!100 (closed)
• User doc: Include settings sync feature in Web IDE doc (!166775 - merged)

The problem

The Web IDE uses VSCode's Settings Sync functionality to store a user's preferences across sessions and web browsers. Currently, the REST API that backs the Web IDE's Settings Sync doesn't support storing user profiles. We should document which data objects aren't supported yet by the Web IDE's Settings Sync API.

Acceptance Criteria

• Document technical documentation for the Settings Sync current REST API as part of the Web IDE development docs.
• Provide a user-facing feature summary which includes what VSCode Settings Sync features are currently not supported within the Web IDE docs.