Add Feature Comparison with systemd upstream

[wmanley]

This may be helpful:

• To indicate which areas to work on
• For writing unit files that will work with rustysd

It will need to be kept up to date when new features are added. This should be fairly straightforward as typically it will involve replacing a ❌ with a ✔️. Sometimes a caveat may also need adding to the notes column.

It will also need to be kept up to date with new systemd releases. In this case the generation script can be re-run an a 3-way merge performed.

Hopefully this means that the maintenance cost of this documentation is low enough that the value outweighs the maintenance cost.

[KillingSpark]

This is great! It paints a somewhat dark picture, so I might prefix it with some explanation along the lines of "These sections naturally show just crosses, because they are out of scope". Nonetheless this is a very important document for anyone who might be considering using rustysd.

It should be possible to include existing features into the generation process, similarly to how you insert question marks if the term is found by git grep. Could be a dict indexed by feature name, containing the icon and the text. If no entry is in the dict the default X will be inserted.

Would you be willing to add something like this? If not I will add it to my todo list :)

[KillingSpark]

Nvm, just did it. I just realized that this is a very good way to find stuff that is still missing without having to read the docs all the time. Thanks again!