Update or remove https://archives.dynare.org/DynareWiki/EquationsTags

The information there seems outdated or redundant, but it is still linked from the manual.

changed milestone to %5.x

added documentation label

mentioned in commit JohannesPfeifer/dynare@ce09ef11

mentioned in merge request !1901 (closed)

closed with commit 21f3e120

mentioned in commit 6271a96b