refactor: Add CustomReSTDispatcher as a base class of custom dispatchers

[tk0miya]

Feature or Bugfix

• Refactoring

Purpose

• To create custom reST dispatcher easily, this adds CustomReSTDispatcher
  class as a base class of custom dispatchers.
• As a preparation of Add intersphinx role #9062 and Intersphinx role (2) #9822, this adds a base class for custom dispatchers.

[jakobandersen]

@tk0miya, this is "just" the dispatcher base class part of the other PR(s), right? May I merge it so I can rebase #9822?

[tk0miya]

Okay, let's move forward. Withdrawing this.