Revamp TOP README and implement a TODO list
Review tracers.rst by adding snippets of source files
Add drafthtml target for Makefile related to draft building tag for sphinx-build to
display todo items at the top of the guide homepage

Misc:

• Use dedicated :file: role instead of ... for file or directory
• Update BibTeX entries of NEMO reference publications by using :title: role
• Dump of RELEASE_NOTES.rst
• New file todos.rst to list all items declared with todo directive
• Modify conf.py to add a conditional test for draft tag