[buffer][doc] Avoid having apply and other buffer-strategy members in the doc
They are not relevant for the user. They should only be relevant for users developing
their own strategies, but that should be documented in the (upcoming) strategy concepts
[buffer] rename buffer_circle to point_circle to have it consistent with other strategies,
and buffer is already in the namespace. same for buffer_square -> point_square
[buffer] rename buffer_side to side_straight to make it consistent with other buffer strategies
and buffer is already part of the namespace
[buffer][doc] Enhancements, cross-links, doc-fixes
[buffer][doc] fix buffer_with_strategies example
[buffer][doc] Add documentation for side_straight, extended doc for buffer with strategies
[buffer][doc] Enhance doc