Fix Issue 10297 - Memory safe D spec is out of date

https://issues.dlang.org/show_bug.cgi?id=10297

* Link to safe spec from front page, move `safed.html` article link to safe spec.
* Replace 'Proscribed Forms' section with link to @safe function attribute - this avoids duplication.
* Introduce @safe, @trusted.
* Add note about array bounds checks.