Fix block references + added BlockContext

Found that the BlockBundle link is missing on the cmf docs homepage under bundles. And then found some other references to be fixed.

Directly updated the block bundle docs to include the BlockContext.

The references update should fix:
- the BlockBundle link is missing on the cmf docs homepage under bundles
- previous and next topics are not working for the block chapters
- numbers in front of the chapters and paragraphs should now be removed, to be in sync with the other Symfony docs
- removed the reference part from the block bundle introduction as we now have an index