Iss17 sdcard & iss 22

[a2800276]

• added sd card description
• added getting started pics

supercedes #25 and includes everything in there, namely:

Structure from Issue #22:

• spread out content into directory structure (idf, uPython, fpga, ...)
• updated links
• edited typos, etc.

[a2800276]

superseded by #29