LIBREDB_NO_BANNER is undocumented anywhere outside .env.example

## What

The `LIBREDB_NO_BANNER` env var, which suppresses the first run startup banner, is documented only inside .env.example. Nowhere else under docs/ mentions it, even where the banner itself is discussed at length.

## Where

docs/RANCHER.md:70 and :118 both discuss the first run banner, for example "The banner is printed once, by the container that ran the first start." and "Read the generated credentials from the first-run banner", but neither mentions that it can be turned off.

.env.example, around line 487:

```
# Set to "1" or "true" to print nothing. Has no effect when embedded in libredb-platform.
# LIBREDB_NO_BANNER=1
```

## Expected

Someone reading docs/RANCHER.md's banner discussion learns that it can be suppressed, without already knowing to check .env.example.

## How to fix

Add a short sentence to docs/RANCHER.md, near where the banner is discussed, naming `LIBREDB_NO_BANNER` and what it does.

## How to check

docs/RANCHER.md names the var.

Open docs/RANCHER.md and read the first-run banner discussion around lines 70 and 118. Add a short sentence naming LIBREDB_NO_BANNER and explaining that it suppresses the startup banner, then verify that docs/RANCHER.md mentions the variable.

Written by the indexing model from the issue text.