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 stronger wording here, e.g.:

```
In case where the user can guarantee there are no other references to the same array they can make careful use of $(OBJREF assumeSafeAppend, assumeSafeAppend) to circumvent this behavior.
```

And yeah, a link alone is fine, no need to add examples here. :) They should read the full docs of `assumeSafeAppend` rather than reaching for copy-pasting from this page,.