STYLE: Make doc module consistent with Sphinx conventions

[jhlegarreta]

Make the documentation module consistent with Sphinx conventions:

• Rename the module to docs.
• Host the configuration file and index file in docs/sources. Change the makefile SOURCEDIR variable accordingly.
• Change accordingly the sphinx-apidoc command output path argument, and the configuration file location in the .readthedocs.yaml config file.
• Change the relative paths to the files in the _static folder.
• Move the Markdown bundle naming and tutorial files and the associated screenshots to the docs/sources folder so that all files can be recursively included from the source directory.

[jhlegarreta]

Will need to be rebased after PR #180 and the GHA workflow file be adapted: https://github.com/SlicerDMRI/whitematteranalysis/pull/180/files#diff-03efc769b870804394632e45d7885272b44c16939517fb31c9d7c614d2ffae57R18