[P3] Document non-goals: arbitrary-precision/rational/complex, OOP, char type, multi-line comments

[ecto]

The hyperpolyglot ml column ends up with ~40 none cells. Many
of those reflect deliberate design choices, not gaps:

• Arbitrary-precision integers, rational numbers, complex numbers
• Objects, inheritance, methods, field access on user types
• A distinct Char type (single-character Str is the choice)
• Multi-line/block comments (only ; line comments)

Today there's no page that says these are intentional. A short
Non-goals / by design page would make those nones read as
decisions rather than unfinished work, and steer new users to
the idiomatic alternatives:

• Use Int/Float; no bignum.
• Use ADTs + functions; no OOP.
• Use Str; no Char.
• Use ; line comments.

Acceptance criteria

• New page (e.g. web/src/pages/concepts/non-goals.loon) linked
  from the home/concepts nav.
• Each non-goal lists the idiomatic replacement.