README: new About section and minor wordsmithing

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
README: documentation building instruction update

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
docs: section title display fix

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>