Add steps to enable weekly renewal on FreeBSD

Hello!

Weekly renewal isn't working on FreeBSD 13.1.

The `/usr/local/etc/periodic/weekly500.certbot-3.9` cronjob immediately exits and will not run unless some values are added to `/etc/periodic.conf`.

I believe the fix is to add a step to the documentation on these pages:

- https://certbot.eff.org/instructions?ws=apache&os=freebsd
- https://certbot.eff.org/instructions?ws=nginx&os=freebsd

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## Enable automatic renewal

Add these lines to `/etc/periodic.conf`:

```
# Let's Encrypt: Weekly renewal
weekly_certbot_enable="YES"
weekly_certbot_service="apache"
weekly_certbot_post_hook="service apache24 restart"
# weekly_certbot_deploy_hook="/tmp/a.sh"
# weekly_certbot_custom_args="--force-renewal"
```

- - -

For the Nginx version of the docs, swap out these two lines in the above block:

```
weekly_certbot_service="nginx"
weekly_certbot_post_hook="service nginx restart"
```

- - -

Also, the docs have a line that could use an edit. FreeBSD doesn't have `systemd`.

> The Certbot packages on your system come with a cron job ~~or systemd timer~~ that will renew your certificates automatically before they expire.