fix: improve password validation flow (#15132)

Refers to #14984 

Currently, password validation is done backend side and is not explicit
enough so it can be painful to create first users.
We'd like to make this validation easier - but also duplicate it
frontend side to make it smoother.

Flows involved : 
- First user set password
- New user set password
- Change password

---------

Co-authored-by: BrunoQuaresma <bruno_nonato_quaresma@hotmail.com>

docs/reference/api/authorization.md

@@ -178,6 +178,53 @@ curl -X POST http://coder-server:8080/api/v2/users/otp/request \
 | ------ | --------------------------------------------------------------- | ----------- | ------ |
 | 204    | [No Content](https://tools.ietf.org/html/rfc7231#section-6.3.5) | No Content  |        |
 
+## Validate user password
+
+### Code samples
+
+```shell
+# Example request using curl
+curl -X POST http://coder-server:8080/api/v2/users/validate-password \
+  -H 'Content-Type: application/json' \
+  -H 'Accept: application/json' \
+  -H 'Coder-Session-Token: API_KEY'
+```
+
+`POST /users/validate-password`
+
+> Body parameter
+
+```json
+{
+	"password": "string"
+}
+```
+
+### Parameters
+
+| Name   | In   | Type                                                                                   | Required | Description                    |
+| ------ | ---- | -------------------------------------------------------------------------------------- | -------- | ------------------------------ |
+| `body` | body | [codersdk.ValidateUserPasswordRequest](schemas.md#codersdkvalidateuserpasswordrequest) | true     | Validate user password request |
+
+### Example responses
+
+> 200 Response
+
+```json
+{
+	"details": "string",
+	"valid": true
+}
+```
+
+### Responses
+
+| Status | Meaning                                                 | Description | Schema                                                                                   |
+| ------ | ------------------------------------------------------- | ----------- | ---------------------------------------------------------------------------------------- |
+| 200    | [OK](https://tools.ietf.org/html/rfc7231#section-6.3.1) | OK          | [codersdk.ValidateUserPasswordResponse](schemas.md#codersdkvalidateuserpasswordresponse) |
+
+To perform this operation, you must be authenticated. [Learn more](authentication.md).
+
 ## Convert user from password to oauth authentication
 
 ### Code samples

docs/reference/api/schemas.md

@@ -6446,6 +6446,36 @@ If the schedule is empty, the user will be updated to use the default schedule.|
 | `dormant`   |
 | `suspended` |
 
+## codersdk.ValidateUserPasswordRequest
+
+```json
+{
+	"password": "string"
+}
+```
+
+### Properties
+
+| Name       | Type   | Required | Restrictions | Description |
+| ---------- | ------ | -------- | ------------ | ----------- |
+| `password` | string | true     |              |             |
+
+## codersdk.ValidateUserPasswordResponse
+
+```json
+{
+	"details": "string",
+	"valid": true
+}
+```
+
+### Properties
+
+| Name      | Type    | Required | Restrictions | Description |
+| --------- | ------- | -------- | ------------ | ----------- |
+| `details` | string  | false    |              |             |
+| `valid`   | boolean | false    |              |             |
+
 ## codersdk.ValidationError
 
 ```json