Additional solution documentation intended to facilitate developer onboarding

[vazmigue]

While each function in the code is properly documented, it's hard is to get a full grasp about which actions the solution is taking at every step and why they are being taken. Even for technical roles, you might not get a full view of the solution in a short amount of time. This limits customers that are willing to build on top of this solution. Some issues that we have identified while on-boarding technical roles into PCA:

• Lack of knowledge about call centers and its terminology. We are talking about the metadata that you might find in audio files, CLMs, audio channels, hertzs, narrobands, etc
• Lack of knowledge about Amazon Transcribe. Understand their different APIs (Standard, Call-Analytics) and what’s the criteria for invoking one or the other. Same applies for understanding what information is transcribed by each API mode (and when you need Comprehend for things like Sentiment Analysis)

This task will add an additional PCA_walkthrough.md file in which each component of the architecture is explained and documented.