Add deprecation notice for TemplatedConfigLoader and ConfigLoader in the documentation

[ankatiyar]

          > The example look good to me. One comment about the structure of this note, should we place OmegaConfigLoader before TemplatedConfigLoader? I think we should recommend omegaconf for templated value

We can do a separate ticket for moving the section for TemplatedConfigLoader below OmegaConfigLoader and perhaps adding Deprecation notice in the docs for both ConfigLoader and TemplatedConfigLoader. What do you think?

Originally posted by @ankatiyar in #2896 (comment)

To do :

• Add deprecation notice to ConfigLoader and TemplatedConfigLoader
• Link the deprecation notice to the Migration guides (Migration guide for switching to OmegaConfigLoader #2699)
• Move section for OmegaConfigLoader above the TemplatedConfigLoader section in Advanced Configuration docs