Document various deprecation processes

Right now we only have these dev docs talking about the deprecation process:

• https://docs.moodle.org/dev/Deprecation
• https://docs.moodle.org/dev/String_deprecation

This issue is about to complete missing information about other deprecations that have some differences with the general deprecation process. Mainly:

• classes deprecation (see MDLSITE-6729).
• events deprecation (we have already deprecated some, look for MDL-18592 or MDL-63211 for example).
• external functions deprecation (there is only a 1-line note in the current docs, pretty vague).
• capabilities deprecation (to be decided, but there are a number of cases awaiting for it, see MDL-55580).
• behat steps deprecation (it already exists, but, needed for plugins, see MDL-73669).
• adhoc tasks
• scheduled tasks
• JavaScript (MDLSITE-6080)
• CSS (MDL-78334)
• <<any other thing that we consider is not covered by any of the above, feel free>>

Ciao