docs: add PyFreeDB

[AlbertLucianto]

What is this Python project?

Python library that provides common and simple database abstractions on top of Google Sheets.

1. Provide a straightforward key-value and row based database interfaces on top of Google Sheets.
2. Serve your data without any server setup (by leveraging Google Sheets infrastructure).
3. Support flexible enough query language to perform various data queries.
4. Manually manipulate data via the familiar Google Sheets UI (no admin page required).

What's the difference between this Python project and similar ones?

Comparisons are elaborated in Alternative Analysis section.

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