docs: avg_speed and si_constants examples added to docs

docs/users_guide/examples/si_constants.md

@@ -0,0 +1,44 @@
+---
+tags:
+  - Physical Constants
+  - Text Formatting
+---
+
+# `si_constants`
+
+!!! example "[Try it on Compiler Explorer](https://godbolt.org/z/????)"
+
+The next example presents all the seven defining constants of the SI system. We can observe
+how [Faster-than-lightspeed Constants](../framework_basics/faster_than_lightspeed_constants.md)
+work in practice.
+
+```cpp title="si_constants.cpp" linenums="1"
+--8<-- "example/si_constants.cpp:28:34"
+```
+
+As always, we start with the inclusion of all the needed header files. After that, for
+the simplicity of this example, we
+[hack the character of quantities](../framework_basics/character_of_a_quantity.md#hacking-the-character)
+to be able to express vector quantities with simple scalar types.
+
+```cpp title="si_constants.cpp" linenums="1"
+--8<-- "example/si_constants.cpp:36:"
+```
+
+The main part of the example prints all of the SI-defining constants. While analyzing the output of
+this program (provided below), we can easily notice that a direct printing of the quantity provides
+just a value `1` with a proper constant symbol. This is the main power of the
+[Faster-than-lightspeed Constants](../framework_basics/faster_than_lightspeed_constants.md) feature.
+Only after we explicitly convert the unit of a quantity to proper SI units we get an actual numeric
+value of the constant.
+
+```text
+The seven defining constants of the SI and the seven corresponding units they define:
+- hyperfine transition frequency of Cs: 1 Δν_Cs = 9192631770 Hz
+- speed of light in vacuum:             1 c = 299792458 m/s
+- Planck constant:                      1 h = 6.62607015e-34 J s
+- elementary charge:                    1 e = 1.602176634e-19 C
+- Boltzmann constant:                   1 k = 1.380649e-23 J/K
+- Avogadro constant:                    1 N_A = 6.02214076e+23 1/mol
+- luminous efficacy:                    1 K_cd = 683 lm/W
+```