Provide further instructions and examples for logging in

**What would you like to be added or enhanced**
Mention in the docs that the username and password need to be base64 encoded when calling `/login`

**Why is this needed**
I was confused when I received an error attempting to login using my custom immudb credentials:
```json
{
"code": 3,
"message": "illegal base64 data at input byte 4"
}
```
It led me to believe that my credentials were wrong, or they were possibly hardcoded in immugw, only when I thought to try a base64 decode on `aW1tdWRi` did I figure out the issue.

Start with the documentation for the /login endpoint and review its current authentication instructions. Document that the username and password must be base64 encoded, include a representative example, and ensure the reported login error is explained so users can authenticate successfully.

Written by the indexing model from the issue text.