secondary-sub: add secondary-sub-* osd style options for secondary subtitles, like add secondary-sub-color - #18491

…tle, instead of always referencing primary subtitle style.

Effectively adding options for, among others;
secondary-sub-color
secondary-sub-outline-color
secondary-sub-back-color
Make default secondary subtitle color gray

Commit 36b7be1 added a full set of secondary-sub-* style options
(secondary-sub-font, secondary-sub-outline-size, secondary-sub-align-x,
etc.) mirroring the existing sub-* style options, but only documented
secondary-sub-color, secondary-sub-outline-color, and
secondary-sub-back-color. This adds options.rst entries for the
remaining secondary-sub-* options, each pointing back to its sub-*
counterpart for the full description, per the existing --secondary-sub-*
documentation convention in this file.

AI assistance disclosure: an AI assistant (Claude) was used to compile
the list of documented vs. undocumented secondary-sub-* options and to
draft the options.rst text and this commit message. I reviewed and
adjusted the wording before committing, and take full responsibility
for the change and its correctness.