📖 DOC: Standardize usage of `versionadded` directive

[edan-bainglass]

This PR proposes the following:

• Major releases should discard all versionadded admonitions of previous releases

• New features that can be subsectioned should be subsectioned as follows:

  Header
  ------
  
  .. versionadded:: <tag>
  
  Content

• When subsectioning is not possible, use the following format:

  .. versionadded:: <tag>
    Short, single-sentence summary
  
    Longer description

The proposed standard is implemented in this PR.