Update the documentation

- Improve build section by describing new build options
- Add a section about driver APIs and basic usage examples
- Add a section about mutexes and semaphores
- Add a section about begin/end inference callbacks
- Add a brief section about driver implementation design
- Fix markdown title/subtitles structure
- Small fixing of typos and rewording

Also add .gitignore

Change-Id: I7216a2b72b0dfaa605620f4344da205235339ddb