probe_as_z_home: Initial implementation and documentation

[nickweedon]

This pull request adds the ability to perform multiple probes when issuing a G28 command. More specifically it calls into the 'probe' logic to execute a probe in a way similar to what would be performed during bed mesh creation. Since it uses the same logic this means that the resulting probe also applies the same calc_probe_z_average method to the result.

The benefit of this feature is it allows users with less than perfect probing hardware to still have an unsupervised and reliable homing experience which is crucial to ensure print quality and preventing failed prints due to an incorrect Z homing. Bad homing can even lead to extruder jams and waste significant amounts of the 3D printer user's time. This is a feature that has been requested many times with one such example being Discourse: multiple-samples-on-home-z.

To configure this tool a new configuration section called [probe_as_z_home] should be placed anywhere in the config before the either [safe_z_homing] or [homing_override].

The implementation is completely decoupled from any particular homing or probing method and does not modify any existing code.

[vgdh]

great!

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