Conform to expectations regarding directories for docs and for test-data

[dalito]

The template inherited a slightly unconventional directory layout from linkml-project-cookiecutter.

In most python and many other projects, some files are stored at different locations. So the template breaks the expectations. We we should fix that.

• Source files for the documentation are currently src/docs. We should move them to docs/. Note that docs/ is currently git-ignored as it is used as build-directory for the documentation. The new build dir should be docs/_build (need to check if this name is typical for mkdocs; it is for sphinx).

• Test data used in unit tests are currently in src/data/examples/ but typically stored under tests/....