feat: getting started section

[adidottxt]

This PR replaces the current introduction file with a Getting Started section that contains:

1. A brief introduction (the plan is to add this video in as well – we do not want to put the video into this repo though, so maybe we need a YouTube channel and set this video to private?)
2. A quickstart section
3. A concepts section that briefly goes through, at a top level, some basics that are relevant to Porter that may not be clear to users (especially to non-technical founders) – Linear does something similar here.