Document vcpkg installation and possible variants

Nobody has claimed this yet.

The installation process is only briefely explained in the docs as `git clone`, `bootstrap-vcpkg.bat` and, optionally, `vcpkg integrate install`.

This is fine for default setup, but there is more details that really need to be covered in the documentation.

Some of obvious questions"

1. Shall I copy `vcpkg.exe` anywhere?
1. Can I copy `vcpkg.exe` anywhere (eg. `c:\bin\vcpkg.exe` which I have in `PATH`) or it has to stick to location where it was bootstrapped?
1. Can I control location where `vcpkg.exe` downloads/installs packages in?
1. What is the `{vcpkg}\buildtrees` directory for?
1. What is the `{vcpkg}\installed` directory for?
1. What is the `{vcpkg}\downloads` directory for?
1. What is the `{vcpkg}\packages` directory for?
1. What is the relation between the working copy where `vcpkg.exe` was bootstrapped and location where packages are installed?
1. Having `C:\vcpkg-1\vcpkg.exe` and `C:\vcpkg-2\vcpkg.exe`, do I maintain two sets of packages (two installation prefixes, in Unix speak)
1. Do such multiple working copies affect the user-wide integration?
1. Does use of such multiple working copies require use of the CMake toolchain file only?
1. Having the multiple working copies, can I mix integration modes? For example
* `C:\vcpkg-1\vcpkg.exe` is user-wide integrated
* `C:\vcpkg-2\vcpkg.exe` is used via the CMake toolchain file only

So far, user has to fish for (correct?) answers in the issues archive and compile the information from **scraps**, hoping the bigger picture generated is correct.

For example, https://github.com/Microsoft/vcpkg/issues/1681#issuecomment-324160402 suggests to clone a second copy...

## References

#3044
#3052

Start with the existing installation documentation and review the referenced issue 1681 comment, #3044, and #3052. Document the executable location, package directories, installation prefixes, multiple working copies, and integration modes requested in the issue. Done means the listed questions are answered together rather than through scattered issue comments.

Written by the indexing model from the issue text.