Add PostgreSQL backend documentation

[Copilot]

Documents the newly added PostgreSQL backend implementation alongside existing SQLite, MySQL, and Redis backends.

Changes

• docs/source/includes/_backends.md: Added PostgreSQL section with constructor signature, options (WithPostgresOptions, WithApplyMigrations, WithBackendOptions), and schema table descriptions
• backend/postgres/README.md: Added migration instructions following existing backend pattern
• README.md, docs/source/index.html.md: Updated backend enumeration to include PostgreSQL
• .gitignore: Excluded docs build artifacts

Usage

import "github.com/cschleiden/go-workflows/backend/postgres"

b := postgres.NewPostgresBackend("localhost", 5432, "user", "password", "workflows",
    postgres.WithApplyMigrations(true),
    postgres.WithBackendOptions(
        backend.WithLogger(logger),
    ),
)

Original prompt

There is now a new postgres backend. Update the documentation to include a description of this new backend implementation. Follow what's already there for the sqlite and mysql backend.

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