Improve discoverability of suggested external tools and explain when they are useful

When building a codelist, there are suggested tools to make building a codelist easier. The tools shown depends on the coding system you are building the codelist in. Where they are suggested, there isn't a description of what they're useful for. There's no way to see the suggested tools outside the builder.

It would be useful to have a section in the docs that describes all of the tools that are suggested, the coding systems they can be used for and a short description of why they are useful.

Start by reviewing the codelist builder to identify every suggested external tool and the coding systems where each appears, then check the existing documentation structure for a suitable location. Add a documentation section with each tool, its supported coding systems, and a short explanation of when it is useful; done means the full set of suggested tools can be understood without opening the builder.