dns and https setup for (www.)borgbackup.org

## DNS zone borgbackup.org:
```
@ IN A
www IN CNAME borgbackup.github.io.
```
Notes:
- first entry: CNAME does not work for @ thus one needs to lookup the IP(s) used by github and configure same IP(s) as A record(s) in DNS.
- second entry: github complains "incorrectly configured" if not like shown.

## Github repo settings for borgbackup/borgbackup.github.io:
```
Custom domain: www.borgbackup.org
```
Notes:
- it is **not** possible to fill in multiple values in the form field, like:
`www.borgbackup.org borgbackup.org`
- we used `www.borgbackup.org` - that redirects `b.o --> www.b.o`, which is wanted.
- if one used `borgbackup.org`, it would redirect like: `www.b.o --> b.o`

```
X Enforce https
```
Notes:
- this will fetch a letsencrypt cert
- sadly, it is only for `www.borgbackup.org` and not also for `borgbackup.org`, so the latter will have a mismatching cert.