Use a standardized folder to store user data (Docker)

### Problem Description

AGH currently uses two folders to store its data: `/opt/adguardhome/conf` that stores the lone configuration file and `/opt/adguardhome/work` that stores the remaining user data including databases and filters text files.

This file structure feels very convoluted, because there are multiple folders and a muddy hierarchy. The presence of a "work" folder feels particularly redundant.

```sh
❯ tree /opt/appdata/adguardhome/
/
├── config
│ └── AdGuardHome.yaml
└── workdir
├── data
│ ├── filters
│ │ ├── 1.txt.old
│ │ ├── 1620456986.txt
...
│ │ └── 740910348
│ ├── sessions.db
│ └── stats.db
└── leases.db

4 directories, 59 files
```

### Proposed Solution

A singular `/config` folder that stores user data in a single folder with a less confusing hierarchy, including databases in the root (not inside another `data` folder).

This _same_ could be used for non-Docker installations as well, just `/opt/adguardhome/`.

### Alternatives Considered

### Additional Information

I'd also request the use of better file names to denote their purpose, e.g. `sessions.db` → `web_sessions.db`.