Make local check-symbols.sh testing possible and add documentation

Currently it depends on suitable Gitlab variables being set. This MR changes it so that manually setting only the required variables is possible.

How to test:

# How to test locally:
# Inside the kicad-symbols folder, run
#     export BASE_SHA=$(git rev-parse master)
#     export TARGET_SHA=$(git rev-parse HEAD)
#     export CI_PROJECT_DIR=$(pwd)
#     export CI_BUILDS_DIR=../
#     ../kicad-library-utils/tools/gitlabci/check_symbols.sh