Add handbooks

We now have a means of listing handbooks and other material in the Anthology, via the `` block:

```


https://2023.eacl.org
2023.eacl.handbook.pdf


```

I have collected a few of these on the server under `~/files/handbooks.`. These need to be added to the event blocks. We also should track down others.

Listing:

```
$ cd ~/files/handbooks
$ ls */*.handbook.pdf
acl/2014.acl.handbook.pdf
acl/2016.acl.handbook.pdf
acl/2022.acl.handbook.pdf
eacl/2023.eacl.handbook.pdf
emnlp/2017.emnlp.handbook.pdf
naacl/2013.naacl.handbook.pdf
naacl/2015.naacl.handbook.pdf
```

Locate the Anthology event blocks for the seven handbook paths listed under ~/files/handbooks, then inspect how existing links are represented. Add links for those PDFs and check for other available handbooks, verifying that each link points to the corresponding file.

Vom Indexierungsmodell aus dem Issue-Text verfasst.