Renames files /doc/training-guides/developer-training-guide

Renames files with tags 'bkxxx' and 'chxxx' with
cleaner tags 'bk' and 'ch'.

Partial-bug: #1338684

Change-Id: I0a06f1e5e8c3e1492c77763deb9e65e22ccc7b6f
Merge "Renames files /doc/training-guides/developer-training-guide"