Docs: add help make goal and enhance the docs

[v1v]

This will help to show what each make goal does

## UI

$ make -f .ci/Makefile
build                Build the project
composer             Run composer
help                 Display this help text
install              Install the agent
prepare              Build docker image for building and testing the project
test                 Test the project

$ make -C packaging help
build                Build docker image for the packaging
deb-info             Show the deb package metadata
deb-install          Install the deb installer to run some smoke tests
deb                  Create the deb installer
help                 Display this help text
info                 Show the package metadata for all the installers
package              Create all the installers
rpm-info             Show the rpm package metadata
rpm-install          Install the rpm installer to run some smoke tests
rpm                  Create the rpm installer
tar                  Create the tar.gz
version              Show the fpm version

[apmmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #93 opened]

• Start Time: 2020-07-20T13:01:33.122+0000

• Duration: 10 min 40 sec

Test stats 🧪

Test	Results
Failed	0
Passed	78
Skipped	0
Total	78