Renames the matrix_mautrix_SERVICENAME_* placeholders in the common
mautrix bridges guide to matrix_bridge_mautrix_SERVICENAME_*, and fixes
prose in the matrix-bridge-steam role that referred to the old variable
prefix and to a misspelled (underscore-styled) systemd service name.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>