docs: document that license validation works offline (#24013)

## What

Documents that Coder license keys are validated locally using
cryptographic signatures and do not require an outbound connection to
Coder's servers. This is a common question from customers evaluating
Coder for air-gapped environments.

## Changes

- **`docs/admin/licensing/index.md`**: Added an "Offline license
validation" section explaining that license keys are signed JWTs
validated locally with no phone-home requirement.
- **`docs/install/airgap.md`**: Added a "License validation" row to the
air-gapped comparison table, confirming no changes are needed for
offline license validation and linking to the licensing docs.

## Why

While the air-gapped docs state that "all Coder features are supported"
offline, there was no explicit mention that the license itself doesn't
require connectivity. This is a frequent question from
security-conscious and air-gapped customers.

---------

Co-authored-by: blink-so[bot] <211532188+blink-so[bot]@users.noreply.github.com>
Co-authored-by: Matyas Danter <mdanter@gmail.com>

docs/admin/licensing/index.md

@@ -7,6 +7,14 @@ features, you can [request a trial](https://coder.com/trial) or
 
 ![Licenses screen shows license information and seat consumption](../../images/admin/licenses/licenses-screen.png)
 
+## Offline license validation
+
+Coder license keys are signed JWTs that are validated locally using cryptographic
+signatures. No outbound connection to Coder's servers is required for license
+validation. This means licenses work in
+[air-gapped and offline deployments](../../install/airgap.md) without any
+additional configuration.
+
 ## Adding your license key
 
 There are two ways to add a license to a Coder deployment: