docs: project template design - root and custom directories

Describe project template design with respect to different directories in resources/template-domain/, e.g. resources/practicalli/

Existing content:
https://practical.li/clojure/clojure-cli/projects/templates/design-templates/

root

All directories and files are copies as they are into the root directory of the new project.

Use for any files that do not need template rules, e.g. the :transform in the template.edn for the project.

Custom directories

Use custom directories when rules should be applied to files and directories when copying.

• build
• src
• test

src and test paths have the domain namespace added, using the top key value, e.g. [["src" "src/{{top/file}}"]] will create files under src/practicalli/

The main src and test namespace can be set using the file key value, e.g. using {{main/file}} src/practicalli/gameboard.cljandtest/practicalli/gameboard_test.clj`