Add READMEs for Upgrade section (#4526)

Author: mconcas

Detectors/Upgrades/IT3/README.md

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+# IT3
+Upgraded version of the ITS that includes upgraded truly-cylindrical inner barrel.
+
+# Run the full simulation
+Provided O2 has been compiled with upgrades enabled, it is possible to simulate ITS 3 geometry within the `o2-sim` executable.
+
+## Simulation
+IT3 module is enabled via the `-m IT3` parameter.
+In case of `PIPE` to be enabled, the size of the beam pipe is automatically scaled to what is foreseen for the upgrades.
+
+Typical command to generate MC data:
+```bash
+o2-sim -m PIPE IT3 [...]
+```
+
+## Reconstruction
+Currently, the reconstruction is driven the `macro/run_trac_alice3.C`, which takes as input the hits generated by simulation. Hits can be smeared upon request (see `kUseSmearing`).

Detectors/Upgrades/PostLS4/IT4/README.md

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+# IT4
+At the moment the IT4 name is a placeholder for barrel detector.
+
+# Run the full simulation
+Provided O2 has been compiled with upgrades enabled, it is possible to simulate IT4 geometry within the `o2-sim` executable.
+
+## Simulation
+IT4 module is enabled via the `-m IT4` parameter.
+In case of `PIPE` to be enabled, the size of the beam pipe is automatically scaled to what is foreseen for the upgrades.
+
+Typical command to generate MC data:
+```bash
+o2-sim -m PIPE IT4 [...]
+```
+
+## Reconstruction
+Currently, the reconstruction is driven the `macro/run_trac_alice3.C`, which takes as input the hits generated by simulation. Hits can be smeared upon request (see `kUseSmearing`).

Detectors/Upgrades/README.md

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+<!-- doxy
+\page refDetectorsITSMFT ITSMFT
+/doxy -->
+
+# Upgrades
+This is a top page for detectors not included in Run 3 layout.
+
+# Installation
+
+To enable the compilation of this section it is required to prepend the `ENABLE_UPGRADES=ON` to the installation command, for instance:
+
+```bash
+ENABLE_UPGRADES=ON aliBuild build O2 --defaults o2
+```
+
+<!-- doxy
+* \subpage refIT3
+* \subpage refPostLS4
+/doxy -->