README: overhaul for uniformity

- add overview of functionality
- add a supported hardware overview
- move Golioth Features above Local set up
- Separate Stream from State
- Add Stream example JSON
- Add LightDB State Example JSON
- Clarify LightDB State functionality
- update OTA instructions for Packages/Cohorts/Deployments
- Move pipelines higher in the document
- Reverence the pipelines section from the Stream section
- Move hardware variants lower in the document
- Simplify the hardware variants section
- rename opening section
- make all headings after the first subheadings
- fix mention of LightDB Stream
- properly style admonitions for GitHub
- add section for additional sensors/components
- remove hardware variations sections
- add mention of use cases and link to projects site
- add section for asking questions

Signed-off-by: Mike Szczys <[email protected]>

Author: szczys