Merge: doc/commands: extract CmdCode superclass

Provides a common super class to factorize code rendering services for doc commands.

Changes:
* renamed `CmdComment::render` to `render_comment` to avoid name conflicts with multiple inheritance
* extracted `CmdCode` command to factorize code related services
* introduced new class `CmdEntityCode`, in place of the old `CmdCode` class
* updated nitx/nitweb clients
* updated tests

Pull-Request: #2609