docs: update references to use ds_mysql.hpp umbrella header

- README.md Quick Start: replace 4 individual includes with #include "ds_mysql.hpp"
- docs/QUICKREF.md Key Headers: add ds_mysql.hpp as first entry with
  'Umbrella header' description
- docs/DEVELOPMENT.md: update new-header guidance to reference the three
  top-level headers that feed the umbrella; update example snippet to use
  #include "ds_mysql.hpp"

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Pierre-Luc Gagné

README.md

@@ -18,10 +18,7 @@ DSMySQL provides a **header-only** library for building type-safe SQL queries an
 ## Quick Start
 
 ```cpp
-#include "ds_mysql/mysql_database.hpp"
-#include "ds_mysql/sql.hpp"
-#include "ds_mysql/column_field.hpp"
-#include "ds_mysql/varchar_field.hpp"
+#include "ds_mysql.hpp"
 
 using namespace ds_mysql;
 

docs/DEVELOPMENT.md

@@ -27,7 +27,7 @@ DSMySQL is a header-only library. All functionality lives in `lib/include/ds_mys
 1. Create a new header under `lib/include/ds_mysql/`
 2. Use `#pragma once`
 3. Put all code in the `ds_mysql` namespace
-4. Include it transitively through `ds_mysql/sql.hpp` or `ds_mysql/mysql_database.hpp` if appropriate
+4. Include it transitively through `ds_mysql/mysql_database.hpp`, `ds_mysql/mysql_metadata.hpp`, or `ds_mysql/sql_extension.hpp` if appropriate (those three are pulled in by the `ds_mysql.hpp` umbrella header)
 
 ## Adding Unit Tests
 
@@ -37,7 +37,7 @@ DSMySQL is a header-only library. All functionality lives in `lib/include/ds_mys
 
 ```cpp
 #include <boost/ut.hpp>
-#include "ds_mysql/sql.hpp"
+#include "ds_mysql.hpp"
 
 using namespace boost::ut;
 using namespace ds_mysql;

docs/QUICKREF.md

@@ -41,12 +41,13 @@ DSMySQL/
 
 | Header | Purpose |
 |--------|---------|
+| `ds_mysql.hpp` | **Umbrella header** — include this to get everything |
 | `ds_mysql/mysql_database.hpp` | MySQL connection & queries |
 | `ds_mysql/sql.hpp` | SQL query builder |
 | `ds_mysql/sql_extension.hpp` | MySQL-specific extensions |
+| `ds_mysql/mysql_metadata.hpp` | MySQL information_schema types |
 | `ds_mysql/column_field.hpp` | Typed column descriptors |
 | `ds_mysql/schema_generator.hpp` | Schema generation |
-| `ds_mysql/mysql_metadata.hpp` | MySQL information_schema types |
 | `ds_mysql/mysql_config.hpp` | Connection configuration |
 | `ds_mysql/varchar_field.hpp` | Fixed-length string type |