Align doc references with the trunk-5.5 release, mirroring the
convention used for the trunk-5.4 tag.
- Bump cross-repo URL links (nuttx / nuttx-apps / vendor_*
/ packages_demos / packages_fe_examples / docs) from
/blob|tree/trunk/ to /blob|tree/trunk-5.5/
- Update `repo init -b trunk` commands and surrounding prose in
the Ubuntu quick-start guides to use `-b trunk-5.5`
- Update patch-version naming example in README from trunk-5.2.1
to trunk-5.5.1
Historical release notes (v5.2.md, v5.4.md), commit-SHA pinned
links in the chip-porting guides, the PR template's link to
CONTRIBUTING (kept on `trunk` so contributors always see the
latest guidelines), and descriptive prose about the `trunk`
branch itself are left untouched.
zhangxiaowei16