doc: improve documentation of objects (6)

## PixelHit

The digitised pixel hits after processing in the detector's front-end electronics. The object allows the storage of both the

time and signal value. The time can be stored in an arbitrary unit used to timestamp the hits. The signal can hold different

kinds of information depending on the type of the digitizer used. Examples of the signal information is the "true"

information of a binary readout chip, the number of ADC counts or the ToT (time-over-threshold).

The digitised pixel hits after processing the [PixelCharge](#pixelcharge) in the detector's front-end electronics. The object

allows the storage of both the time and signal value. The time can be stored in an arbitrary unit used to timestamp the hits.

The signal can hold different kinds of information depending on the type of the digitizer used. Examples of the signal

information is the "true" information of a binary readout chip, the number of ADC counts or the ToT (time-over-threshold).

The exact type of the time and signal values depends on the digitizer module used, for which the

[module documentation](../08_modules/_index.md) is to be consulted.

Main parameters:

- The pixel corresponding to the hit

  ([`getPixel()`](https://allpix-squared.docs.cern.ch/reference/classallpix_1_1PixelHit.html#a1da68ec967f329bc389024fc6967c4ad))

  and its index

  ([`getIndex()`](https://allpix-squared.docs.cern.ch/reference/classallpix_1_1PixelHit.html#a0859f268a4f7a69aad7ca6cce742f03b))

- The related [PixelCharge](#pixelcharge)

  ([`getPixelCharge()`](https://allpix-squared.docs.cern.ch/reference/classallpix_1_1PixelHit.html#a3c0794a68c71e930913438f151afcf66))

  and PixelPulse, if any

  ([`getPixelPulse()`](https://allpix-squared.docs.cern.ch/reference/classallpix_1_1PixelHit.html#a2cb314929cced5f6f82ebe7e4a3016bd))

- The signal of the hit

  ([`getSignal()`](https://allpix-squared.docs.cern.ch/reference/classallpix_1_1PixelHit.html#ae9fc38ec4d0aa41aff2eb019ce2ce236))

- The time information of the hit in local and global coordinates

  ([`getLocalTime()`](https://allpix-squared.docs.cern.ch/reference/classallpix_1_1PixelHit.html#a99fad43a7ba5d87f7f774d26d81c7dc2),

   [`getGlobalTime()`](https://allpix-squared.docs.cern.ch/reference/classallpix_1_1PixelHit.html#a1d541acb02366d25691b2114613a8521))

For more details refer to the [code reference](https://allpix-squared.docs.cern.ch/reference/classallpix_1_1PixelHit.html).