Add documentation for Vector approx matcher

[claremacrae]

PR #1499 was included in 2.7.2 - but not added to the documentation during that release, as far as I can see from looking at the "Files changed" tab in the changes since the previous release:

v2.7.1...v2.7.2

The addition for this PR was ApproxMatcher, which does not appear in any documentation.

If this is added to the documentation, please include the following text, with a blank line either side:

> [Introduced](https://github.com/catchorg/Catch2/issues/1499) in Catch 2.7.2.