Document AGH file system

@mercyground, please file a separate issue about that. I wanted a single place that explains the AdGuard Home file system for a long time, so why not now, heh.

_Originally posted by @ainar-g in https://github.com/AdguardTeam/AdGuardHome/issues/4208#issuecomment-1024477146_

As an addition for documentation I wrote up this for the OpenWrt Wiki. It was part of my how can I make things smaller investigation and might be worth adding to your wiki as reference or explanation?

https://openwrt.org/docs/guide-user/services/dns/adguard-home#data_files

(copied here for quick reading)
```
Data Files:

The AdGuardHome/data folder contains the following.

root@OpenWrt:/opt/AdGuardHome/data# ll -h
drwxr-xr-x 3 root root 512 Oct 29 09:42 ./
drwxrwxrwx 4 root root 736 Oct 30 09:06 ../
drwxr-xr-x 2 root root 800 Nov 2 09:52 filters/
-rw-r--r-- 1 root root 45.4M Nov 2 20:42 querylog.json
-rw-r--r-- 1 root root 8.9M Oct 29 09:00 querylog.json.1
-rw-r--r-- 1 root root 32.0K Oct 30 05:28 sessions.db
-rw-r--r-- 1 root root 4.0M Nov 2 21:00 stats.db

querylog files : These are your DNS queries. Can be removed.

sessions.db : active logins to AGH currently. This can be deleted but you will need to relog back in.

stats.db : Your statistics database. can purge but you will lose your statistics data.

The filters folder contains all your filter downloads. Purge if it is full but it will re-download your filters.
If your filters are too large for your diskspace you will have to disable large filters and restrict their usage.

The AdGuardHome/agh-backup folder contains the previous version of AGH.
This also can be removed if space is at a premium.
```