refactor: 💥 q_* UDL renamed to _q_*

We had some fun exploring the STD UDLs for potential collisions,
we have learnt our lesson and know how to proceed.
Now is high time to start behaving and obeying C++ rules.

docs/quick_start.rst

@@ -4,22 +4,22 @@ Quick Start
 Here is a small example of possible operations::
 
     // simple numeric operations
-    static_assert(10q_km / 2 == 5q_km);
+    static_assert(10_q_km / 2 == 5_q_km);
 
     // unit conversions
-    static_assert(1q_h == 3600q_s);
-    static_assert(1q_km + 1q_m == 1001q_m);
+    static_assert(1_q_h == 3600_q_s);
+    static_assert(1_q_km + 1_q_m == 1001_q_m);
 
     // dimension conversions
-    static_assert(1q_km / 1q_s == 1000q_m_per_s);
-    static_assert(2q_km_per_h * 2q_h == 4q_km);
-    static_assert(2q_km / 2q_km_per_h == 1q_h);
+    static_assert(1_q_km / 1_q_s == 1000_q_m_per_s);
+    static_assert(2_q_km_per_h * 2_q_h == 4_q_km);
+    static_assert(2_q_km / 2_q_km_per_h == 1_q_h);
 
-    static_assert(2q_m * 3q_m == 6q_m2);
+    static_assert(2_q_m * 3_q_m == 6_q_m2);
 
-    static_assert(10q_km / 5q_km == 2);
+    static_assert(10_q_km / 5_q_km == 2);
 
-    static_assert(1000 / 1q_s == 1q_kHz);
+    static_assert(1000 / 1_q_s == 1_q_kHz);
 
 .. admonition:: Try it on Compiler Explorer
 
@@ -46,7 +46,7 @@ of basic library features::
     int main()
     {
       using namespace units::physical::si::literals;
-      Speed auto v1 = avg_speed(220q_km, 2q_h);
+      Speed auto v1 = avg_speed(220_q_km, 2_q_h);
       Speed auto v2 = avg_speed(si::length<international::mile>(140), si::time<si::hour>(2));
       Speed auto v3 = quantity_cast<si::metre_per_second>(v2);
       Speed auto v4 = quantity_cast<int>(v3);