Feature: add additional projects which are not listed in HACS

[legovaer]

Describe the issue:

Great to see that the first submission for an additional project has already been created in #43. At this point, the projects.yaml file is an automatically generated file that contains metadata of projects that are available in HACS. In order to add additional projects, that have no association with HACS, we would need to manually update the projects.yaml file after each update. This would be one hell of a maintenance task.

Therefore I'd suggest to store all metadata, coming from HACS in a dedicated YAML file. Then we will create another YAML file where we can add metadata of additional projects, which are not included in the auto-generated metadata.

Then it's just a matter of customising the GitHub Action that will generate the final projects.yaml file by merging both the YAML files with the project metadata.