Add short top-level descriptions of each category of competencies

[MakisH]

While it would be great to have the descriptions of the competencies directly on the competencies.md, I still feel uneasy adding them before the review is over, as I feel we are still changing wordings etc. Let's not create additional duplication (we will already have duplication in the de-RSE position and the reviewed version of the paper).

This PR:

• Removes the TODOs for adding description summaries to each skill. The titles are anyway often specific enough.
• Adds top-level summaries of each category of competencies.

I tried to keep the descriptions short and avoid as much as possible any tricky terms.

Closes #15.