[LLD][ELF] Add initial LLD LinkerScript docs page

[LLD][ELF] Add initial LLD LinkerScript docs page

LLD implements Linker Scripts as they are described in the GNU ld manual.
This description is far from a specification, with the only true reference
the GNU ld implementation, which has undocumented behaviour that can vary
from release to release.

To make it easy for people to switch between linkers we try to follow GNU
ld implementation details wherever possible. We reserve the right to make
our own decisions where the undocumented GNU ld behaviour is not
appropriate for LLD. We don't have a place to document these decisions and
it can be difficult for users to find out this information.

This file is a statement of the LLD implementation policy and will contain
intentional deviations from GNU ld.

The first patch that will add concrete details to this file is D75724

Differential Revision: https://reviews.llvm.org/D75921