Update plugin development documentation for mautic 5

[technik-kvz]

We are currently trying to migrate our plugin for mautic 4 to mautic 5, but face the problem, that we don't know were to start (e.g. how the Plugin Directory Structure changed).

At least a link to https://github.com/mautic/mautic/blob/5.x/UPGRADE-PHP-TO-TWIG-TEMPLATES.md and https://github.com/mautic/mautic/blob/5.x/UPGRADE-5.0.md would be nice.

[RCheesley]

Hi there, thanks for raising this as an issue! Would you like to make a PR to add this?

If you clone down the repo, the readme explains how to set up locally (or you can use Gitpod).

If you cd into the /docs directory, you can use the command make link to create a link file, which can be referenced using the syntax is gives you on completion. Commit that + the changes and all will be good to go!