Centralize and update docs for floating point comparisons

The new docs mention that Approx is deprecated and should not be
used, and explain the reasons behind it.

Closes #1444

docs/matchers.md

@@ -150,49 +150,8 @@ are:
 
 > `WithinRel` matcher was introduced in Catch2 2.10.0
 
-
-`WithinAbs` creates a matcher that accepts floating point numbers whose
-difference with `target` is less than the `margin`.
-
-`WithinULP` creates a matcher that accepts floating point numbers that
-are no more than `maxUlpDiff`
-[ULPs](https://en.wikipedia.org/wiki/Unit_in_the_last_place)
-away from the `target` value. The short version of what this means
-is that there is no more than `maxUlpDiff - 1` representable floating
-point numbers between the argument for matching and the `target` value.
-
-**Important**: The WithinULP matcher requires the platform to use the
-[IEEE-754](https://en.wikipedia.org/wiki/IEEE_754) representation for
-floating point numbers.
-
-
-`WithinRel` creates a matcher that accepts floating point numbers that
-are _approximately equal_ with the `target` with tolerance of `eps.`
-Specifically, it matches if
-`|arg - target| <= eps * max(|arg|, |target|)` holds. If you do not
-specify `eps`, `std::numeric_limits<FloatingPoint>::epsilon * 100`
-is used as the default.
-
-
-In practice, you will often want to combine multiple of these matchers,
-together for an assertion, because all 3 options have edge cases where
-they behave differently than you would expect. As an example, under
-the `WithinRel` matcher, a `0.` only ever matches a `0.` (or `-0.`),
-regardless of the relative tolerance specified. Thus, if you want to
-handle numbers that are "close enough to 0 to be 0", you have to combine
-it with the `WithinAbs` matcher.
-
-For example, to check that our computation matches known good value
-within 0.1%, or is close enough (no different to 5 decimal places)
-to zero, we would write this assertion:
-```cpp
-    REQUIRE_THAT( computation(input),
-        Catch::Matchers::WithinRel(expected, 0.001)
-     || Catch::Matchers::WithinAbs(0, 0.000001) );
-```
-
-
-> floating point matchers live in `catch2/matchers/catch_matchers_floating.hpp`
+For more details, read [the docs on comparing floating point
+numbers](comparing-floating-point-numbers.md#floating-point-matchers).
 
 
 ### Miscellaneous matchers