[docs] Revamp the installation guide (#1877)

This tries to revamp/restructure the installation guide. It's renamed to
"Getting Started" as it now contains a few more things than just the
installation, especially the deployment considerations which we didn't
use to spell out as much ahead of time.

Installation is now a section with the guides on their own. I've removed
a bit of redundancy like the user creation. I also removed the rogue
reverse proxy section in the Docker guide and lifted that into the
reverse proxy section.

docs/getting_started/user_creation.md

@@ -0,0 +1,50 @@
+# Creating users
+
+Regardless of the installation method, you'll need to create some users. GoToSocial currently doesn't have a way for users to be created through the web UI, or for people to sign-up through the web UI.
+
+Using the CLI, you can create a user:
+
+```sh
+$ gotosocial --config-path /path/to/config.yaml \
+    admin account create \
+    --username some_username \
+    --email some_email@whatever.org \
+    --password 'SOME_PASSWORD'
+```
+
+In the above command, replace `some_username` with your desired username, `some_email@whatever.org` with the email address you want to associate with your account, and `SOME_PASSWORD` with a secure password.
+
+If you want your user to have admin rights, you can promote them using a similar command:
+
+```sh
+$ gotosocial --config-path /path/to/config.yaml \
+    admin account promote --username some_username
+```
+
+Replace `some_username` with the username of the account you just created.
+
+!!! info
+    When running these commands, you'll get a bit of output like the following:
+
+    ```text
+    time=XXXX level=info msg=connected to SQLITE database
+    time=XXXX level=info msg=there are no new migrations to run func=doMigration
+    time=XXXX level=info msg=closing db connection
+    ```
+
+    This is normal and indicates that the commands ran as expected.
+
+## Containers
+
+When running GoToSocial from a container, you'll need to execute the above command in the conatiner instead. How to do this varies based on your container runtime, but for Docker it should look like:
+
+```sh
+$ docker exec -it CONTAINER_NAME_OR_ID \
+    /gotosocial/gotosocial \
+    admin account create \
+    --username some_username \
+    --email someone@example.org \
+    --password 'some_very_good_password'
+```
+
+If you followed our Docker guide, the container name will be `gotosocial`. Both the name and the ID can be retrieved through `docker ps`.