Merge branch 'feature/categorize_examples_doc_new' into 'master'

Organize examples and API documentation

Organize / categorize growing number of code examples and API documentation,  ref. https://gitlab.espressif.cn:6688/idf/esp-idf/issues/10

This merge request is intended to replace most of https://gitlab.espressif.cn:6688/idf/esp-idf/merge_requests/421. The rest of changes submitted in https://gitlab.espressif.cn:6688/idf/esp-idf/merge_requests/421 will follow.

1. Moved examples to category folders, removed numbers from example names
2. Modified `buid_examples.sh` script to handle new locations of examples
3. Moved api-name.rst files to category folders
4. Updated documentation TOC (`docs/index.rst`)
5. Updated links to examples in API documentation

See merge request !430