docs(duckdb): add public dataset tutorials

[mariadb-AlekseiBukhalov]

What

Two step-by-step tutorials for the DuckDB storage engine, plus the supporting doc updates:

• NYC Taxi Trips — load one month of TLC Yellow Taxi Parquet with run_in_duckdb() + read_parquet(), keep the taxi zone lookup in ENGINE=InnoDB, then join DuckDB and InnoDB in a plain SELECT. Follows the structure of the ClickHouse taxi tutorial so the two are comparable.
• OWID CO₂ Emissions — load a commit-pinned CSV revision, run aggregations and a LAG() window function.
• docs/mariadb-duckdb-incompatibilities.md — documents TIMESTAMPDIFF() under a new "Not translated by the pushdown layer" heading.
• README.md — new "Tutorials" section linking both.

Docs only: 4 files, 602 insertions, 0 deletions, no code touched.

Testing

The tutorials are treated as a spec: every SQL block was extracted programmatically from the
markdown, executed against a live server, and every expected-output block was compared cell by
cell with the actual result.

Environment:

Server	11.4.13-MariaDB, built from this branch (RelWithDebInfo)
Base	11.4 @ 90bea36bfad
Plugin	ha_duckdb.so, SHOW ENGINES → DUCKDB / YES, maturity gamma
DuckDB submodule	v1.5.2
Settings	duckdb_allow_run_in_duckdb=ON, local_infile=ON

Data sets (verified by sha256 against the URLs in the tutorials):

File	sha256
owid-co2-data.csv	7f78e2b218ce4bb8c538bbec04fdc9a7982e8d40bff972e650df603899edd5f6
yellow_tripdata_2024-01.parquet	c4d59da7bbc8abaeeeb1727947ee93d9891a71acb42854bd80db1571b2030510
taxi_zone_lookup.csv	1a99e105092230f8620f301edcca7f80d3080642ff404d28ed957d3fa222c8ed

Result — 13 output blocks compared, all matched:

--- OWID (50,411 rows loaded) ---
OK  COUNT(*) + MIN/MAX/COUNT(DISTINCT)
OK  top-10 emitters
OK  decade aggregation
OK  LAG() China year-over-year

--- NYC (2,964,624 trips + 265 zones) ---
OK  COUNT(*) total_trips          OK  duration buckets
OK  MIN/MAX pickup_datetime       OK  zones count + information_schema engines
OK  AVG(tip_amount)               OK  cross-engine join, airport trips by borough
OK  avg cost by passenger_count   OK  pickup zones ranking
OK  busiest pickup hours

Also checked:

• Cross-engine pushdown: with duckdb_log_options='DUCKDB_QUERY' the error log shows
  DuckDB: cross-engine pushdown with 1 external table(s) and the pushed-down statement, as the
  tutorial claims.
• LOAD DATA LOCAL INFILE into InnoDB, and information_schema.TABLES reporting
  taxi_zones / InnoDB and trips / DUCKDB.
• The tutorial's claim about out-of-range timestamps is measured, not guessed: 18 rows fall outside
  January 2024, and AVG(tip_amount) is 3.34 with and without the month filter.
• DROP DATABASE cleanup for both tutorials.
• Every relative link in the new files resolves.

[CLAassistant]

All committers have signed the CLA.

[drrtuy]

untested ACK