Fix sphinx documentation errors

 - make sure all pages are linked from somewhere
 - prohibit double indexing of source code modules
 - remove old unused offers (we use GIT, no need to keep old files)
 - use local version of Oscar logo

There are still couple issues left (probably work in progress):

```
/home/imatusov/workspace/oscar/docs/source/releases/v0.6.rst:99: ERROR: Unknown interpreted text role "function".
/home/imatusov/workspace/oscar/docs/source/releases/v0.6.rst:16: ERROR: Indirect hyperlink target "upgrading advice" (id="upgrading-advice") refers to target "upgrading", which does not exist.
/home/imatusov/workspace/oscar/docs/source/releases/v0.6.rst:12: ERROR: Unknown target name: "deprecated features".
/home/imatusov/workspace/oscar/docs/source/howto/how_to_configure_stock_messaging.rst:5: WARNING: undefined label: availabiliy_policies (if the link has no caption the label must precede a section header)
/home/imatusov/workspace/oscar/docs/source/topics/key_questions.rst:79: WARNING: unknown document: /howto/how_to_configure_tax
```