Add icons for key features

[ee7]

The icons for key features are now available, and configlet lint now requires that each key feature has a valid icon value.

This PR sets an initial icon for each key feature - feel free to change them however you want. You can see the list of available icons here. Note that an icon can be used regardless of its name.

With the initial state of this PR, the key features look something like the below.

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Cross-platform
C is portable and can run on almost any platform and chipset.

General purpose
C can be used for a wide variety of workloads from IoT to game engines.

Performant
C has been consistently rated as one of the highest performing languages for energy and time.

Stable
The C standard values stability and backwards compatibility, C programs can run for decades.

Powerful
C allows both lower and higher level functionality, making it very powerful.

Efficient
C allows swift implementation using constructs that map efficiently to machine instructions.

[wolf99]

The change looks good to me.

Though maybe the double use of the fast icon indicates that we could rethink the efficient feature. Either to reword to align a little with another icon, or to replace it with a different feature (maybe portable or embeddable).

If replacement is agreed then the efficient content text could be added into the fast content text so as not to lose that aspect completely.

What do you think?

[ee7]

Though maybe the double use of the fast icon indicates that we could rethink the efficient feature.

I agree. I couldn't see how to avoid the duplicate icon here.

or to replace it with a different feature (maybe portable or embeddable).

I lean towards this. But perhaps portable overlaps too much with the existing "Cross-platform" wording?

I guess widely-used is possible too. Maybe just look at key features on other tracks and steal one that fits?

If you're okay with the icons otherwise - should we just merge this PR, and then you can follow-up however you want?