Some documentation love

[azmeuk]

• Splitted index.rst in another introduction.rst file, because it was too loaded for a homepage.
• I moved all the *.txt files from the source directory to the documentation directory, renamed them to .rst and fixed the titles so it produce a coherent hierarchy.
• The documentation can be built with tox -e docs
• I switched references to www.zodb.org to https://zodb-docs.readthedocs.io as the certificate for zodb.org is not valid. (See zodb.org certificate #284)

[azmeuk]

@jamadden Thanks for reviewing this one. As usual it seems that the RTD doc does not build :)

[jamadden]

Sorry, I have no ability to update https://zodb-docs.readthedocs.io. The only owner is @jimfulton