Dynamic/static arrays: Fix some doc errors

Split out the less controversial fixes from https://github.com/D-Programming-Language/dlang.org/pull/623:

Fix some documentation errors, around dynamic array semantics and static array syntax.
- Add a note about the need for assumeSafeAppend in order to regrow a dynamic array.
- Add examples for static arrays as lvalues (for copying and setting).
- Soften an over-strong claim in ctod.dd.
- Also tweak some heading levels and such.
I would add a note what `d = t` does. You could add an example, e.g.:

```D
int[] e = new int[3];
e = t;
e[0] = 10;
assert(t[0] == 10);
```