doc: Add documentation small improvements

This patches adds:
  * Rename codeline-management.rst to codeline_management.rst to keep
    consistency
  * Style external links
  * Center all images
  * Fix link to documentation build instructions

Issue-id: SCM-4394
Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Change-Id: Iadfefce84f72fd22352790acc607e3512cda4424