[DOCS] Fix errors in CONTRIBUTING.md (#1911)

* Fix git clone URL to point to contributor's fork

* Fix fork workflow in contributing guide

Add missing git fetch upstream before branching from
upstream/master. Clarify fork step with link to
GitHub's fork guide.

* Replace broken pip setup with uv workflow

Remove pip command that used uv-only flags (--all-extras,
--requirement pyproject.toml). Add explicit venv creation
and activation steps before make install-dev.

* Fix rebase branch from develop to master

* Fix git push to target fork instead of upstream

* Fix step numbering to continue from 6 through 8

* Remove stray code fence closings in style guide

* Fix sub-bullet indentation under motivation list

* Fix spelling and grammar throughout contributing guide

* Update outdated external links throughout contributing guide

* Use consistent imperative form in bug report bullets