Improve documentation for plugin translation, advice a way to ensure unique translation keys

[lutzhelm]

Mirodor allows the translation of plugins by providing a translations object via the config property of the plugin object, e.g.

https://github.com/ProjectMirador/mirador-image-tools/blob/9556d016f238f7bf2fa09f0f155907391a1543ff/src/plugins/miradorImageToolsPlugin.js#L23-L25

1. We should add this info to the plugin documentation in the wiki.
2. Any two different plugins might accidentally use the same key for different translations, resulting in one plugin overriding the other plugin's translation. Additionally, we might add new translation keys to the core Mirador that already exist within any third party plugin, resulting in overriding the Mirador core translation. Therefore, we should advice the use of plugin specific prefixes or namespaces to ensure unique translation keys.