Add more relevant information in the 'compilers' section

https://conda-forge.org/docs/maintainer/knowledge_base.html#compilers

Currently, the 'Compilers' section is incomplete and unhelpful.

Refer: https://github.com/conda-forge/conda-forge.github.io/issues/893#issuecomment-634881487

The link mentioned in the comment by @isuruf doesn't exist as conda-forge doesn't use wiki anymore.
(https://github.com/conda-forge/conda-forge.github.io/issues/893#issuecomment-634915638)

**Suggested Solution:**

The following list should be added to the compiler section:

**Activation recipes**
1.GCC - https://github.com/conda-forge/ctng-compiler-activation-feedstock
2. Clang, OSX - https://github.com/conda-forge/clang-compiler-activation-feedstock
3. gfortran, OSX - https://github.com/conda-forge/gfortran_osx-64-feedstock

**Implementation packages**
1. GCC - https://github.com/conda-forge/ctng-compilers-feedstock
2. gfortran, OSX - https://github.com/conda-forge/gfortran_impl_osx-64-feedstock
3. clang
https://github.com/conda-forge/llvmdev-feedstock
https://github.com/conda-forge/libcxx-feedstock
https://github.com/conda-forge/clangdev-feedstock
https://github.com/conda-forge/openmp-feedstock
https://github.com/conda-forge/tapi-feedstock

4. OSX - https://github.com/conda-forge/cctools-and-ld64-feedstock