Minor issues when building the docs

[javiereguiluz]

As reported on https://symfony.com/doc/build_errors:

Errors for "doctrine-mongodb-bundle/master":

cookbook/registration_form.rst:: WARNING: document isn't included in any toctree

first_steps.rst:105: WARNING: undefined label: cookbook-mongodb-field-types (if the link has no caption the label must precede a section header)