init: oauth docs (#2565)

* init: oauth docs

* chore: update directories

* update: feedback

docs/install/oauth.md

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+# GitHub OAuth
+
+By default, Coder is accessible via built-in authentication. Alternatively, you
+can configure Coder to enable logging in through GitHub OAuth. See below for
+configuration steps.
+
+## Step 1: Configure the OAuth application in GitHub
+
+First, [https://developer.github.com/apps/building-oauth-apps/creating-an-oauth-app/](register a GitHub OAuth app). GitHub will ask you for the following Coder parameters:
+
+- **Homepage URL**: Set to your Coder domain (e.g. `https://coder.domain.com`)
+- **User Authorization Callback URL**: Set to `https://coder.domain.com/api/v2/users/oauth2/github/callback`
+
+Note the Client ID and Client Secret generated by GitHub. You will use these
+values in the next step.
+
+## Step 2: Configure Coder with the OAuth credentials
+
+Navigate to your Coder host and run the following command to start up the Coder
+server:
+
+```console
+coder server --oauth2-github-allow-signups=true --oauth2-github-allowed-orgs="your-org" --oauth2-github-client-id="8d1...e05" --oauth2-github-client-secret="57ebc9...02c24c"
+```
+
+Alternatively, if you are running Coder as a system service, you can achieve the
+same result as the command above by adding the following environment variables
+to the `/etc/coder.d/coder.env` file:
+
+```console
+CODER_OAUTH2_GITHUB_ALLOW_SIGNUPS=true
+CODER_OAUTH2_GITHUB_ALLOWED_ORGS="your-org"
+CODER_OAUTH2_GITHUB_CLIENT_ID="8d1...e05"
+CODER_OAUTH2_GITHUB_CLIENT_SECRET="57ebc9...02c24c"
+```
+
+Once complete, run `sudo service coder restart` to reboot Coder.

docs/install/postgres.md

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+# PostgreSQL
+
+Coder ships with a built-in PostgreSQL database, but if you'd like to set up and
+use your own, refer to the following instructions.
+
+For in-depth information, please see the [PostgreSQL
+documentation](https://www.postgresql.org/docs/current/tutorial-start.html).
+
+## Step 1: Install and start PostgreSQL
+
+### macOS with Homebrew
+
+1. Install with Homebrew:
+
+    ```console
+    brew install postgres
+    ```
+
+1. Start PostgreSQL with brew
+
+    ```console
+    brew services start postgresql
+    ```
+
+1. Connect to PostgreSQL:
+
+    ```console
+    psql postgres
+    ```
+
+### Debian/Ubuntu
+
+1. Install PostgreSQL:
+
+    ```console
+    sudo apt-get install -y postgresql
+    ```
+
+1. Start PostgreSQL:
+
+    ```console
+    sudo systemctl enable postgresql
+    sudo systemctl start postgresql
+    ```
+
+1. Connect to PostgreSQL:
+
+    ```console
+    sudo -u postgresql psql
+    ```
+
+## Step 2: Create a database and user for Coder
+
+1. Create the `coderuser` role:
+
+    ```console
+    create role coderuser with login;
+    ```
+
+1. Create a database called `coder` and assign the owner:
+
+    ```console
+    create database coder owner coder;
+    ```
+
+1. Set the password for `coderuser`:
+
+    ```console
+    \password coder # enter password when prompted
+    ```
+
+1. Assign rights to the database to your user:
+
+    ```console
+    grant all privileges on database coder to coderuser;
+    ```
+
+## Using your PostgreSQL database with Coder
+
+To use your Postgres database with Coder, provide the `CODER_PG_CONNECTION_URL`
+variable:
+
+```console
+postgresql://[user[:password]@][networkLocation][:port][/dbname][?param1=value1&...]
+```
+
+Append to `coder server` to start your deployment. For example:
+
+```console
+CODER_PG_CONNECTION_URL="postgres://<databaseUsername>@0.0.0.0/<databaseName>?sslmode=disable&password=<password>" \
+    coder server -a 0.0.0.0:3000 --verbose
+```
+
+> If you [installed Coder manually](install.md), you can add the
+> `CODER_PG_CONNECTION_URL` variable to  `/etc/coder.d/coder.env`.