Add documentation section about how the data pipeline works

[llucax]

We need to add a documentation section that explains how the data pipeline works, including:

• Samples and metrics definitions (https://github.com/frequenz-floss/frequenz-sdk-python/wiki/Glossary)
• Source(s): Microgrid API
• Resampling
• Logical meter
• Component pools (battery, EV charger, etc.)
• Moving window (?)
• Periodic feature extractor (?)