added description of the site_i18n plugin

just the main concept for now
site_i18n: first concept of create_lang_copies()
site_i18n: added safety check to run create_lang_copies only once and record_translations concept
added description of what we want to do in record_translations and do it only once
site_i18n: added remove_generator_translations function

this function removes the (hidden_)translations attribute from article and page generators
preventing them from being generated as they will be generated later in a lang sub-site
site_i18n: give Pelican actual settings, not a module
site_i18n: added __init__.py for plugin infrastructure
site_i18n: changed the concept and description a bit
site_i18n: record_translations -> move_translations

all we need to do is to modify the url of the translation to point to the sub-site
however, it must come from the site root, might conflict with prepending
language code to SITE_URL
site_i18n: updated create_lang_copies for new concept
site_i18n: create_lang_copies -> create_lang_subsites

and added a docstring to the func
site_i18n: actually implement config overriding and lang code prepending
site_i18n: prepend the translation url with _main_site_root first
site_i18n: improve create_lang_subsites docstring
site_i18n: add todos
renamed plugin: site_i18n -> i18n_subsites

this better fits its description: makes i18n subsites
i18n_subsites: chanbge DEFAULT_LANG for each lang loop
i18n_subsites: register move_translations only for Article and Page contents
i18n_subsites: bugfix: _main_site_root did not work as it was never used
i18n_subsites: remove six as we don't use it