Add compilers section to user docs

### Where should the content be added?

_No response_

### What should be added?

Expand on the content in the user faq, create an end-user specific section that describes how to use the compiler toolchains:
* tl;dr; install `compilers` package and go
* explain the different packages `_impl`, vs `activation scripts` vs `symlinks`
* document the new `conda-gcc-specs` package
* refer to the maintainer's compilers & runtimes section. Everything there applies
* stress that recipes added must use the `compiler()` jinja function, that the metapackages and specs are there for end-user convenience and usability.

### Additional information

_No response_