docker.rst and installation_modes.rst are inaccessible in online documentation index

[palisadoes]

The make html command fails in the docs/ directory. This is because the index.rst file doesn't have entries for the docker.rst and installation_modes.rst files.

This needs to be fixed so that the installation process documentation reflects these options.

$ make html
Running Sphinx v1.8.5
loading pickled environment... done
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 1 source files that are out of date
updating environment: [] 0 added, 1 changed, 0 removed
reading sources... [100%] contributing                                                                                                                                                             
looking for now-outdated files... none found
pickling environment... done
checking consistency... /opt/pattoo/pattoo/docs/docker.rst: WARNING: document isn't included in any toctree
/opt/pattoo/pattoo/docs/installation_modes.rst: WARNING: document isn't included in any toctree
done
preparing documents... done
writing output... [100%] index                                                                                                                                                                     
generating indices... genindex
writing additional pages... search
copying static files... done
copying extra files... done
dumping search index in English (code: en) ... done
dumping object inventory... done
build succeeded, 2 warnings.

The HTML pages are in _build/html.