Clarify whether the main goal of the Carpentries is, or isn't, skill-building

### How could the content be improved?

https://carpentries.github.io/instructor-training/01-welcome.html states:

> The main goal of The Carpentries is _**not to teach specific skills**_, per se - although those are covered - but rather, to convey best practices that will enable researchers to be more productive and do better research.

But earlier the same page states that:

> The Carpentries is a global community of volunteer researchers, educators, and others oriented around improving basic computing and data skills for researchers

and most of https://carpentries.github.io/instructor-training/02-practice-learning.html is about how to help learners acquire new skills.

The suggestion here is to make clearer whether or not the main goal of the Carpentries is, or is not, to teach skills. (and if it is, that's not mutually exclusive from conveying best practices as well)

Read the statements in 01-welcome.html alongside the discussion of skill acquisition in 02-practice-learning.html. Clarify the relationship between teaching skills and conveying best practices, then check both pages for consistent wording. Done means the Carpentries' main goal is understandable without suggesting that skill-building and best practices are mutually exclusive.

Written by the indexing model from the issue text.