Doc: Fixed broken ref to Background Modeling section.

Doc: added description of aperture FLUXERR in photometry section.

Doc: Added description on FLUX_PETRO in Photometry section. Re-wrote part of the FLUX_AUTO subsection to better describe "reduced pseudo-radii".

Doc: added Flagging section and flag description tables in Windowed measurement and Weighting sections.

Doc: Added :param: role for SExtractor measurements parameters and modified custom CSS to manage the :param: role.
Doc: Added targetparam class and setup targets in preliminary measurement parameter list.

Doc: removed non-compliant characters in citation labels that would create problems with Sphinx v1.6+ (see https://github.com/sphinx-doc/sphinx/issues/3917).