Add documentation to logging on where to find installer logs

Also send bootstrap & installer logs to same file.

bootstrap/bootstrap.py

@@ -26,7 +26,7 @@ def main():
     logger = logging.getLogger(__name__)
 
     os.makedirs(install_prefix, exist_ok=True)
-    file_logger = logging.FileHandler(os.path.join(install_prefix, 'bootstrap.log'))
+    file_logger = logging.FileHandler(os.path.join(install_prefix, 'installer.log'))
     file_logger.setFormatter(logging.Formatter('%(asctime)s %(message)s'))
     logger.addHandler(file_logger)
 

docs/troubleshooting/logs.rst

@@ -10,6 +10,8 @@ before things went bad, and can help us understand the problem so we can fix it.
 
 TLJH collects logs from JupyterHub, Configurable HTTP Proxy, & from each individual
 user's notebook server. All the logs are accessible via `journalctl <https://www.freedesktop.org/software/systemd/man/journalctl.html>`_.
+The installer also writes logs to disk, to help with cases where the
+installer did not succeed.
 
 .. warning::
 
@@ -17,6 +19,14 @@ user's notebook server. All the logs are accessible via `journalctl <https://www
    a problem you might have, be careful to redact any private information (such
    as usernames) from the snippet first!
 
+.. _troubleshooting/logs#installer:
+
+Installer Logs
+==============
+
+The JupyterHub installer writes log messages to ``/opt/tljh/installer.log``.
+This is very useful if the installation fails for any reason.
+
 .. _troubleshoot_logs_jupyterhub:
 
 JupyterHub Logs