Document how users can add IRC support back themselves

Parent issue: #8573

Once we drop Pidgin, likely its replacement won't support IRC.

On #15816 (closed) we reached the conclusion that it was OK to drop IRC support by default, but we should smooth a bit the transition, in order to lower the noise this will create, and ease the pain on Help Desk.

We could:

• Keep the Pidgin configuration in our image
• Drop the Pidgin persistence feature: otherwise it's visible to everybody and we need to keep the corresponding doc
• Document a way to add and configure Pidgin in a persistent manner
  • Install via Additional Software
  • Make ~/.purple persistent, e.g. by adding a line to persistence.conf or via dotfiles

For the discussion that lead there initially, see #15816 (closed).