Create plugins developer guide

[colinkiama]

This pull request includes documentation updates to the developer guides, specifically adding new sections on plugins and their implementation.

Fixes #65

New sections added to the developer guides:

• source/developer-guides/plugins.rst: Added a new section titled "Plugins" with a table of contents.

Examples and guides for implementing plugins:

• source/developer-guides/plugins/01-type-modules.rst: Added a detailed example on implementing a GLib.TypeModule based plugin in Vala, including code snippets and build/run instructions.
• source/developer-guides/plugins/02-libpeas.rst: Added information about using the Libpeas library for creating a plugin engine, including useful links to documentation and the project repository.

[lw64]

I have some issues I found, maybe it can be addressed in a future PR:

• the plugins page is not linked to from the main developer guides page
• the plugin page is completely empty, a short overview/explanation would be good to have
• the libpeas page could also have a code snippet, not sure if one can be reused from somewhere else or someone needs to write it

these are all very easy issues and could be worked on by new contributors