docs: Annotate the smallest C++ hello example with teaching comments

[jose-compu]

Why

A commented hello path lowers the barrier before reading include/logosdb/logosdb.h.

Task

Pick the smallest C++ (or Python) example and add brief comments for: open DB, put vector+text, search top_k, close/cleanup. Do not change behavior.

Acceptance

• Example still builds/runs
• Comments explain public API usage only

Difficulty

Beginner — comments only.