docs: Add basic docs for semaphore

docs/services/semaphore.md

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+# GoToSocial
+
+[Semaphore](https://www.ansible-semaphore.com/) is a responsive web UI for running Ansible playbooks. Installing it is powered by the [mother-of-all-self-hosting/ansible-role-semaphore](https://github.com/mother-of-all-self-hosting/ansible-role-semaphore) Ansible role.
+
+## Dependencies
+
+This service requires the following other services:
+
+- a [Postgres](postgres.md) database
+- a [Traefik](traefik.md) reverse-proxy server
+
+
+## Configuration
+
+To enable this service, add the following configuration to your `vars.yml` file and re-run the [installation](../installing.md) process:
+
+```yaml
+########################################################################
+#                                                                      #
+# semaphore                                                            #
+#                                                                      #
+########################################################################
+
+semaphore_enabled: true
+semaphore_hostname: semaphore.example.com
+semaphore_admin_password: STRONG_PASSWORD
+semaphore_admin_name: USERNAME
+semaphore_admin_email: user@example.org
+# Key for encrypting access keys in database.
+# It must be generated by using the following command: head -c32 /dev/urandom | base64
+semaphore_access_key_encryption: "PJOfV/7Q+ZDUxo2bgW8dgVbGJ6nNIJgEOyB3hcnVVz4="
+
+########################################################################
+#                                                                      #
+# /semaphore                                                           #
+#                                                                      #
+########################################################################
+```
+
+
+## Usage
+
+After [installing](../installing.md), you can log into you admin account by visiting the URL specified in `semaphore_hostname`.