docs(database): describe auto-increment columns outside the primary key

[roxblnfk]

🔍 What was changed

Primary-key auto-increment was already covered; this documents auto-incremented columns that are not the primary key.

• database/declaration.md: a new "Auto-increment Columns Outside the Primary Key" section with per-driver behaviour — PostgreSQL serial() / bigSerial() / smallSerial() (Adding the ability to use serial without primary key database#106, 2.5.0); MySQL, where since cycle/database 2.23.4 (fix(MySQL): keep auto-increment columns out of the primary key database#275) an autoIncrement: true column stays out of the primary key and needs its own index, written into CREATE TABLE; no support on SQLite and SQL Server.
• annotated/entity.md: entity examples for a PostgreSQL serial column and a MySQL autoIncrement column with a unique index, how the value is read back (RETURNING on PostgreSQL; on MySQL the field stays null until the entity is reloaded), and what happens on other drivers.

Review notes

• 2.23.4 is the pending release-please version (chore(2.x): release 2.23.4 database#276); merge this after that release is out.
• On MySQL the counter can be added only with a new table: adding it to an existing one still fails with error 1075, because the column and its index are added by separate statements. The docs say so without promising a version.

Checklist

• Closes Write about auto increment #175
• How was this tested:
  • PostgreSQL 17 and MySQL 8.0: schema and ORM paths, against cycle/database 2.x (with #275), cycle/orm 2.x, cycle/annotated 4.x
  • SQLite and the column DDL every driver renders