[spec/expression.dd] Improve IsExpression docs

Add Basic, Identifier and Parameter List form subheadings instead of
an ordered list. Better for navigation and grouping relevant things
together. Also use H5 for each is expression kind for anchors.
Move is(Bar == int) example above `== keyword` forms.
Add some missing docs for `== __vector`.
Add example for is(Type == keyword) and link to spec/traits.
Fix: is declaring an identifier also works in StaticAssert.
Make is(Type Identifier) example runnable.
Split is ( Type Identifier : TypeSpecialization ) example in two.
Move is(Bar T == int) example above `== keyword` forms.
Explain `Identifier == keyword` form *result* for super, return,
__parameters.
Split Parameter List form example into 3 and add titles. Define
`struct Tuple(T...)` instead of importing std.tuple for clarity. Use 2
static asserts instead of `assert(0)`.

css:
Add missing h5 style, fixes text size too small.

Author: ntrel

css/style.css

@@ -894,6 +894,11 @@ h4
 {
     font-size: 1.15em;
 }
+
+h5
+{
+    font-size: 1em;
+}
 /*This is to support the "Programming in D" book*/
 h6 {
     font-size: 1em;