Extensions documentation could make need for hx-ext clearer

[ChrisJefferson]

Via google I landed on the manual page for 'response-targets', which did exactly what I want.

For a while I couldn't get it to work, because I'd never used an extension before, and didn't carefully every bit, so I didn't understand how important hx-ext was.

How about, under "install" in each extension, after the "Install" line, adding a line that says something like:

All extensions must be [enabled](@/extensions/_index.md) using [hx-ext](@/attributes/hx-ext.md) before that can be used.

Just to point people towards this important thing they must read (I'm linking to both extensions, and ht-ext, as those are I think the two most useful pages?)

I'm happy to make a PR, just wanted to check what people thought before I bothered going through all the extension pages.