docs: Updating GetStartedGuide

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
docs: Improving Introduction

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
docs: Improving WritingTests

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #29 from avocado-framework/documentation

Documentation