* Improve build instructions

* Merge Linux stuff into one section.
* Provide better info on dependencies
* Provide easier to use instruction for Debian / Ubuntu for native and cross-compile builds
* Capitalize Sentence and put proper punctation.
* Use backticks where appropiate
* Move Hisilicon instruction together with Arm crosscompiling
* Add information how to verify the Vulkan installation on Linux
* Change how to launch examples so it is easier (no copying needed or external resources)
* Add info about benchmark and how to test it on GPU or CPU
* Use `$(nproc)` where appropriate.
* Put some paths in quotes, in case they contain spaces or other weird characters (coma, semicolon, colon, etc).
* Remove some extremely obvious pieces that every developer usually knows.
* Use newer Vulkan SDK in instructions for Linux
* Get rid of `<ncnn-root-dir>`, and just use `ncnn`. It is what git clone does. And any developer will know how to change it if they choose to do it differently.

* Update README.md

* Update how-to-build.md

Co-authored-by: nihui <shuizhuyuanluo@126.com>