Optimizer is not a part of fluid. The `paddle::platform`
was missing.

* first commit

* Update read_source.md

* fix the ending symbol

* fix invalid content link

* update docs about how to add a new operator

This pull request adds "Build using Docker" documentation to the "Getting Started > Install and Build" menu on PaddlePaddle.org

Change the instructions to use https instead of SSH

* Translate the CPU profiling document

* Paragraphing

* refine install docs

* do not remove files

* follow comments

* update

Update the write_docs_en.rst and write_docs_cn.rst to include how to run the documentation viewer tool without Docker
Add dev/contribute_to_paddle_cn.md onto navigation link

* Add documentation of cProfile tools

* Complete doc

* Refine code

* Add English version of Android cross-compiling document

* Add English version of Android cross-compiling document

* Follow comments from Yi-qun and Kavya

* Add documentation of cross-compiling for iOS.

* Correst the typo in documentation of cross-compiling for raspberry pi.

* Set ANDROID_API to 21 when it is specified < 21 for arm64-v8a in build_android.sh.

* Check the input and print the usage in MergeModel.cpp.

* Add cross_compiling_for_raspberry.md

* Update cross_compiling for raspberry pi document

* Some minor edits

* In response to comments from Kavya

* Add the _en suffix

* Add symbolic link from Paddle/CONTRIBUTING.md to doc/howto/dev/contribute_to_paddle_en.md so sphinx can generate the document
* Update CONTRIBUTING.md links so sphinx does not add these links to the TOC
* Removed dev/contribute_to_paddle_cn.md from documentation, since this document is not in sync with Paddle/CONTRIBUTING.md