Improve plugin install/upgrade docs

[dennisameling]

There is a section in the docs called "Install/Upgrade" for plugin developers.

This section has a large deprecation notice in it now:

THIS IS NOW DEPRECATED AND DISCOURAGED FROM USE. USE THE INTEGRATION FRAMEWORK’S MIGRATION INSTEAD.

However, the "Install/Upgrade" section is not only for developers who want to add database migrations. It can also be used in the installation process to check if certain dependencies are installed, etc.

We should merge the docs for "Install/Upgrade" and the new Integration Framework and mention the following:

• You can use onPluginInstall(), onPluginUpdate, etc. to check things during the installation process
• Do you need to add custom tables to the database? Use the new isApplicable() and up() methods that the IntegrationsBundle makes available.

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