Add advice setting for pull request creation from forks

[be-student]

Fixes #1485.

Add advice.macheteCreateFromFork, enabled by default, to suppress the repeated fork warning when set to false.
The shared creation path applies the setting to both github create-pr and gitlab create-mr; repository selection, remote checks, and PR/MR creation remain unchanged.
Document the setting in both commands, regenerate CLI help and the man page, and add a release note.

Validation:

• Before the fix, both disabled-warning cases failed; default and explicitly enabled cases passed.
• tox -e py -- tests/test_fork_advice.py tests/test_github_create_pr.py tests/test_gitlab_create_mr.py -n 2: 43 passed.
• tox -e pep8,isort,mypy,py-docs: passed; mypy checked 112 source files.
• tox -e sphinx-man,sphinx-html,py-docs-check: passed.
• Repository shell checks passed after installing ShellCheck; the optional remark link check was skipped because remark is not installed.
• Per AGENTS.md, flake8 and vulture are left to CI.

Tests use local Git repositories and the existing mocked hosting APIs, with complete command-output assertions for both providers and all three setting states.

[PawelLipski]

PR looks okay! pls rebase & resolve the conflict