yadm: add pages (#13920)

Co-authored-by: Sebastiaan Speck <12570668+sebastiaanspeck@users.noreply.github.com>
Co-authored-by: Wiktor Perskawiec <git@spageektti.cc>

pages/common/yadm-git-crypt.md

@@ -0,0 +1,21 @@
+# yadm git-crypt
+
+> Git Crypt enables transparent encryption and decryption of files in a git repository.
+> See also: `git-crypt`.
+> More information: <https://github.com/AGWA/git-crypt>.
+
+- Initialize repo to use Git Crypt:
+
+`yadm git-crypt init`
+
+- Share the repository using GPG:
+
+`yadm git-crypt add-gpg-user {{user_id}}`
+
+- After cloning a repository with encrypted files, unlock them:
+
+`yadm git-crypt unlock`
+
+- Export a symmetric secret key:
+
+`yadm git-crypt export-key {{path/to/key_file}}`