Preparations for renaming `master` to `stable`

.gitlab-ci.yml

docs/installation/alpine_linux_en.md

docs/installation/arch_linux_en.md

docs/installation/centos7_en.md

docs/installation/debian_based_en.md

docs/installation/debian_based_jp.md

docs/installation/gentoo_en.md

docs/installation/migrating_from_source_otp_en.md

docs/installation/netbsd_en.md

docs/installation/openbsd_en.md

docs/installation/openbsd_fi.md

docs/installation/otp_en.md

mix.exs