[docs] Schema additions, and monodoc template updates.

- Expanded monodoc-ecma schema to support the 'preserve-mono' attribute, and the attribution tag.
- Added preserve-mono to several pieces of documentation that we would like to preserve across future automated documentation imports: CipherSuitesCallback, ServicePointManager, CultureInfo, Environment+SpecialFolder.xml
- XSL template changes for monodoc HTML generation to support <attribution>
[docs] Import of Microsoft BCL Documentation.
The documentation is creative commons licensed.