Document opDollar.
Clarify about opDollar without compile-time argument.
Fix wrong example code.
Move opDollar docs to its own section.

Expand with feedback from Kenji, Don, et al.
Update LDC download links to 0.12.0.

Also rearranges the links to be in the same order as for DMD.
Replace link with FULL_XREF so PDF build can work.
Merge pull request #292 from quickfur/opdollar

Document opDollar (issue 7520)
fix Issue 9665 - Structure constant members can not be initialized if have opAssign

Describe about field initialization behavior.
Merge pull request #402 from klickverbot/ldc-0.12.0

Update LDC download links to 0.12.0.
Fix wrong code in unittest example.
Merge pull request #407 from AndrejMitrovic/FixupUnittest

Fix wrong code in unittest example.
Merge pull request #404 from 9rnsr/fix9665

Issue 9665 - Structure constant members can not be initialized if have opAssign