docs: update the installation instructions – DRAFT

[cedricr]

:wrench: Problem

Attempt to update and simplify the setup instructions.

Fixes https://github.com/MTES-MCT/ecobalyse/issues/1499

:cake: Solution

Adding a dependency to dotenvx might simplify the management of the envvars. For example, instead of having to manually export TRANSCRYPT_KEY=xxx ; ./bin/run-transcrypt.sh, the postinstall step could work automatically.

:rotating_light: Points to watch/comments

I’ve added some comments in the impacted code to open a discussion. They will have to be removed before squashing the PR.