docs: add documentation for admin cli - #832

## Summary

Adds a unified `admin` CLI for self-hosted instance administrators in
both the management and combined binaries.

## User Management

### `admin user change-password`
- Changes a local embedded IdP user's password.
- Selects the user with `--email` or `--user-id`.
- Reads the new password from `--password` or `--password-file`.
- Clears the user's local authentication session so the new password is
required on the next login.
- **Alias:** `admin user set-password`.

### `admin user reset-mfa`
- Resets a local embedded IdP user's MFA enrollment.
- Selects the user with `--email` or `--user-id`.
- Clears TOTP/WebAuthn enrollment data and removes the local
authentication session.
- The user will re-enroll MFA on the next login.

## MFA Management

### `admin mfa status`
- Shows whether local MFA is enabled in the account settings.
- Checks the embedded IdP client configuration and reports whether MFA
is enabled there.

### `admin mfa enable`
- Enables local MFA for embedded IdP users.
- Updates embedded IdP clients and saves the account MFA setting.
- Records an audit event on a best-effort basis.

### `admin mfa disable`
- Disables local MFA for embedded IdP users.
- Updates embedded IdP clients and saves the account MFA setting.
- Records an audit event on a best-effort basis.

## Reverse Proxy Tokens

### `admin token create --name <name> [--expires-in <duration>]`
- Creates a reverse proxy access token.
- Prints the plaintext token once, along with the token ID.
- `--expires-in` supports values such as `24h`, `30d`, or `365d`. If
omitted, the token never expires.

### `admin token list`
- Lists reverse proxy access tokens.
- Shows the token ID, name, creation date, expiration, last-used time,
and revocation status.
- **Alias:** `admin token ls`.

### `admin token revoke <token-id>`
- Revokes a reverse proxy access token.
- Revoked tokens can no longer authenticate reverse proxy instances.

## Reverse Proxy Management

### `admin proxy disconnect-all`
- Lists registered reverse proxy instances and force-marks all connected
instances as disconnected.
- Useful for repairing stale proxy state after an unclean management
server shutdown.
- Prompts for confirmation by default.
- `--dry-run` previews the changes without applying them.
- `--force` skips the confirmation prompt.
- Live proxies may appear again after their next heartbeat, reconnect,
or re-registration.

## Compatibility Commands

### `token ...`
- Deprecated top-level compatibility path.
- Behaves the same as `admin token ...`.
- Retained so existing scripts using `token create`, `token list`, or
`token revoke` continue to work.

## Changes

- Adds reusable `management/cmd/admin` command package.
- Wires `admin` into `netbird-mgmt` and `combined`.
- Adds local user password reset with existing password strength
validation.
- Adds local MFA enrollment reset by clearing Dex TOTP/WebAuthn
credentials and local auth sessions.
- Adds local MFA enable/disable/status helpers for embedded IdP
deployments.
- Moves proxy access token commands under `admin token` for a single
admin-focused CLI entry point.
- Exports `server.ValidatePassword` for reuse by CLI helpers.

## Tests

```bash
go test ./management/cmd/...
go test ./management/cmd/admin ./management/cmd ./combined/cmd
go test ./management/server -run TestValidatePassword
```

Pre-push lint also passed.


### Checklist
- [ ] Is it a bug fix
- [ ] Is a typo/documentation fix
- [x] Is a feature enhancement
- [ ] It is a refactor
- [ ] Created tests that fail without the change (if possible)
- [ ] This change does **not** modify the public API, gRPC protocols,
functionality behavior, CLI / service flags, or introduce a new feature
— **OR** I have discussed it with the NetBird team beforehand (link the
issue / Slack thread in the description). See
[CONTRIBUTING.md](https://github.com/netbirdio/netbird/blob/main/CONTRIBUTING.md#discuss-changes-with-the-netbird-team-first).

> By submitting this pull request, you confirm that you have read and
agree to the terms of the [Contributor License
Agreement](https://github.com/netbirdio/netbird/blob/main/CONTRIBUTOR_LICENSE_AGREEMENT.md).

## Documentation
Select exactly one:

- [x] I added/updated documentation for this change
- [ ] Documentation is **not needed** for this change (explain why)

### Docs PR URL (required if "docs added" is checked)
Paste the PR link from https://github.com/netbirdio/docs here:

netbirdio/docs#832

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

## Release Notes

* **New Features**
* Added self-hosted admin CLI commands for changing passwords, resetting
MFA (including WebAuthn), and managing embedded IdP client MFA
(enable/disable/status).
* Introduced a unified admin command entry point and improved
data-directory handling for embedded IdP storage.

* **Refactor**
* Centralized password strength validation into a shared exported
validator.

* **Tests**
* Added a comprehensive admin command test suite covering password
input, selectors, MFA reset, and client MFA state handling.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->