[ScriptRaccoon]

This PR makes the notation for congruences and cocongruences consistent across all YAML files and content pages.

Notation

• Congruences: a congruence is now always written as $q_1, q_2 : R \rightrightarrows X$, with $R$ for the relation object (previously also $E$, $f, g$, or $p_1, p_2$).
• Cocongruences: a cocongruence is now always written as $j_1, j_2 : X \rightrightarrows C$, with $C$ for the corelation object (previously also $E$).
• Exceptions: where $R$ or $C$ already denotes something else, such as a morphism in Rel or the ring in Proj_fg(R), the relation object keeps a different letter.

Definition as parallel pairs

Some descriptions and proofs treated a congruence as a single morphism $R \hookrightarrow X \times X$ (or a cocongruence as $X \sqcup X \twoheadrightarrow C$). They now all use the parallel pair from the definition in "quotients of congruences" and pass to the induced morphism only when needed.

• Malcev and co-Malcev: the descriptions now define reflexive relations and coreflexive corelations as parallel pairs with a common section or retraction, and link to the definition of (co)congruences.
• Proofs: updated accordingly for Grp (Malcev), Grp_c, Grp_fg, Ban, and Mono.
• Preadditive categories: the proof of the implication "preadditive ⟹ quotients of congruences" has been rewritten as a complete argument.