docs: document the $files and $branches system tables

[jackylee-ch]

$files and $branches are registered system tables — SYSTEM_TABLE_NAMES lists
both, and the 0.3.0 release notes already advertise $files — but the SQL
reference documented only 9 of the 11, so users had no way to learn the column
names without reading system_tables/files.rs. $files is the most useful of the
set for debugging compaction and skew, which makes the omission the more visible
one.

Fix: add a ### $files and a ### $branches section to docs/src/sql.md,
following the existing $tags/$table_indexes shape (purpose sentence, sample
query, column table). Column names and order are taken verbatim from
files_schema() (20 columns) and branches_schema() (2 columns); Arrow types are
rendered in the SQL spelling the surrounding sections already use. The
$branches section also notes the one behavior that sets it apart from its
neighbours: it ignores a $branch_<name> prefix and always reports the base
table's branches.

Docs only — no code or test changes.

[QuakeWang]

The Rust implementation returns "{}" for unpartitioned tables, not NULL, so this description does not match the actual $files output.

[jackylee-ch]

Right, format_partition returns "{}" when there are no partition fields. Fixed.

[QuakeWang]

file_path contains the full data file path, using external_path when present, rather than only the data file name.

[jackylee-ch]

Correct — it is the full path, falling back to split.data_file_path when external_path is absent. Fixed.