fix: Update service provider path and references to bigInt()

[OsitaDNU]

Hi, a few minor updates...

• installation.md: Updated instructions to manually register Sprout's service provider in bootstrap/providers.php instead of bootstrap/app.php.
• installation.md: Changed the tenant_id column type in example migration code from bigInt() to unsignedBigInteger().
• service-overrides.md: Updated the tenant_id column type in the password_reset_tokens table schema from bigInt() to unsignedBigInteger().
• service-overrides.md: Updated the tenant_id column type in the sessions table schema from bigInt() to unsignedBigInteger().