Create new structure for V2 and fluid doc

Implement detection mAP evaluator wrapper and unify label format between SSD loss and mAP evaluator  (#8736)

* Implement mAP evalutor Python interface.

* Fix unit testing and uniy label format between SSD loss and mAP evalutor.

* Update doc.

* add write_new_layers in rst tree

PaddlePaddle.org requires the nav tree to be generated in order to parse the site map.  Since we removed theme in https://github.com/PaddlePaddle/Paddle/pull/8752, this also removed the site navigation menu.  This changelist adds back the site navigation menu. (#8760)

remove doc_theme

disable nccl in memopt fit a line unittest

Add layers for save/load op

fix warning: statement is unreachable

Refine document of build_and_install 

* Refine the doc in detection_output API.

* Refine the doc.

Add guideline for the doc of cmd parameters

v2 API overview refactor doc

fix only shared variables could be declared as static in the device code

Fix vgg16 fc layer size: 4096 rather than 512

Fix fluid distribute build

Add CPU time and MemCopy to the timeline.