Clarify parameter defaults guidance in bgtm and style guide

[miharp]

Problem

The parameter defaults guidance in two pages is unclear and inconsistent,
leading to confusion in module reviews (raised in VoxPupuli IRC).

bgtm.html#parameters covers parameter naming and count but says nothing
about where defaults should live — inline in init.pp or in module hiera
data.

style_guide.html#parameter-defaults recommends module hiera data via
automatic parameter lookup but doesn't distinguish between static cross-OS
values and OS-specific values, doesn't mention the puppet-strings limitation,
and gives no guidance on when Optional[T] = undef is appropriate vs.
misleading.

The result: modules with the same static default value duplicated in both
init.pp and data/common.yaml, and parameters declared Optional that
always receive a value from hiera anyway.

What needs to be documented

Where defaults should live (confirmed by the VoxPupuli PR review guide):

• Static defaults identical across every supported OS → inline in the
  parameter declaration in init.pp. Required because
  puppet-strings#250
  (still open) means defaults in data/common.yaml are invisible to
  generated documentation.
• OS-specific defaults → module hiera data with a per-OS hierarchy.
• Don't duplicate the same value in both places.

Optional[T] = undef — appropriate only when undef is a genuine
runtime value (parameter controls an optional feature). Don't use it simply
to defer a default to Hiera when the parameter will always have a value.

Pages to update

• docs/_openvox_8x/bgtm.md — add parameter defaults guidance under ### Parameters
• docs/_openvox_8x/style_guide.markdown — expand ### Parameter defaults with the two-approach distinction, no-duplication rule, and Optional[T] guidance