Improve to the development setup documentation

[veleek]

Copy the initial section on setting up a development environment from the Core repository developer documentation. Explicitly separate the instructions for doing a manual setup from a devcontainer setup. Move the preferred devcontainer instructions to the top . Added some clarifications and cleanup to the other sections.

Proposed change

Improving developer setup documentation to make it easier for new developers to get started working with the frontend repo.

Type of change

• [ ] Document existing features within Home Assistant
• [ ] Document new or changing features which there is an existing pull request elsewhere
• [X] Spelling or grammatical corrections, or rewording for improved clarity
• [ ] Changes to the backend of this documentation
• [ ] Removed stale or deprecated documentation

Additional information