Documentation for 'getting started' with ACTS is incomplete (was ACTS-443)

Original author Shaun Roe @sroe

Hi, returning after a hiatus to ACTS development, I find my previous scripts to set up no longer work (natch) so I have cause to follow the instructions at

https://gitlab.cern.ch/acts/acts-core/blob/master/getting_started.md#build-lxplus

(I'm on lxplus) ..but I don't get very far:

• The link to LCG91 release appears broken.
• The 'source CI/setup_lcg91.sh' command: I have no alias for path CI, what is this?
• A newer version of git is required before doing any git command.
• The instructions on lxplus tell me to source setup_lcg89.sh, but the preamble tells me to setup lcg91. Which is correct? (in any case the 'CI' path is not set, so neither work)