• Git: add the trademark line suggested by https://git-scm.com/about/trademark

• GitLab: add the trademark line requested by https://about.gitlab.com/handbook/marketing/brand-and-product-marketing/brand/brand-activation/trademark-guidelines/

• GitHub: it is a registered trademark according to the following links: https://github.com/logos https://docs.github.com/en/site-policy/github-terms/github-open-source-applications-terms-and-conditions#githubs-logos https://tmsearch.uspto.gov/bin/showfield?f=doc&state=4808:xvjz5y.2.4

Apply the newly introduced trademarks to the Committer's guide. Also add
a TM symbol immediately after the first reference to the GitLab Name as
requested by
https://about.gitlab.com/handbook/marketing/brand-and-product-marketing/brand/brand-activation/trademark-guidelines/