chore(web-components): add developer quick start page

Updates the docs to include developer quick-start and theming. Modifies the introduction to remove outdated information.

Related Issue(s)

• Fixes #

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@chrisdholt closing as that is the only way to stop notifying reviewers. Please update your PR to point to master following this guide: https://github.com/microsoft/fluentui/pull/31361 and reopen when it's updated. Thanks!