pkgctl auth: add manpages & small improvements

Adds manpages to pkgctl auth, pkgctl auth status and pkgctl auth login.

adds a warning if the token is stored in a (plaintext) file.

Closes #128 (closed)

doc/man/pkgctl-auth-login.1.asciidoc

+pkgctl-auth-login(1)
+====================
+
+Name
+----
+pkgctl-auth-login - Authenticate with the GitLab instance
+
+Synopsis
+--------
+pkgctl auth login [OPTIONS]
+
+Description
+-----------
+
+Interactively authenticate with the GitLab instance.
+
+The minimum required scopes for the token are: 'api', 'write_repository'.
+
+Per default the token is stored in '$XDG_CONFIG_HOME/devtools/gitlab.conf', but
+it can also be exported via env var as 'DEVTOOLS_GITLAB_TOKEN'.
+This allows for it to nicely be supplied via any command (gpg, vault, password
+manager), when the following alias is placed in the shell config:
+
+    $ alias pkgctl='DEVTOOLS_GITLAB_TOKEN="$(command to obtain token)" pkgctl'
+
+Options
+-------
+
+*-g, --gen-access-token*::
+	Open the URL to generate a new personal access token
+
+*-h, --help*::
+	Show a help text
+
+See Also
+--------
+
+linkman:pkgctl-auth-status[1]
+
+include::include/footer.asciidoc[]