Catch unlikely but possible error and head crash when using
Z_PROBE_REPEATABILITY_TEST

Updated documentation in Configuration.h.
Cleaned up and commented some code relating to Z_PROBE_ENDSTOP.
Separated Z_MIN_ENDSTOP and Z_PROBE_ENDSTOP completely.

Documented some additional areas that should be addressed if Z_PROBE is
fully separated from Z_MIN or Z_MAX.
Fixed a documentation error in sanity checks. Servos start at 0 not 1.

- Some messages should not expire with `PROGRESS_MSG_EXPIRE`.
- Simplify conditional for progress bar with sanity checks.
- Rename `messageTick` to `expireStatusMillis` and make it the expire
time.

be.
Added Sanity Check for it.
Added hook so it's enabled.

- Also a small tweak to SanityCheck.h

/SanityCheck.h:107:53: warning: missing terminating ' character
...

- Add `Conditionals.h` with calculated configuration values
- Add `SanityCheck.h` with checks for configuration errors
- Remove equivalent code from all configurations
- Move error checks from some sources to `SanityCheck.h` also
- Fix initialization of count_direction in stepper.cpp