Problem: LDAP documentation does not tell users that an additional PHP extension is required

**Documentation version**

* 2.7, 2.8 (with 2.8 being the priority)

**Relevant documentation pages**

* https://www.accesstomemory.org/docs/2.8/admin-manual/customization/authentication/#enabling-ldap-authentication

**Issue description**

Per this recent discussion in the user forum from 2024-01-29:

* https://groups.google.com/g/ica-atom-users/c/u2vH98xwQV0/m/YcNcO8NGAAAJ

It seems that an additional PHP extension is required to be able configure LDAP: the `php-ldap` package. See:

* https://packages.ubuntu.com/jammy/php-ldap

This can be installed with:

```
sudo apt-get install php-ldap
```

The suggestion of installing this extension came from Santi (via [Slack](https://artefactual.slack.com/archives/C07DPRF6Z/p1706555621576249), also on Jan 29). The forum user who prompted this question later PM'ed to confirm that installing the PHP extension did resolve the issue and allow her to configure the LDAP module.

However, this requirement is not documented anywhere.

**Suggested fix**

Add an IMPORTANT admonition at the beginning of the LDAP configuration instructions clarifying that the `php-ldap` extension must be installed first. Include the install command for Ubuntu, and the link provided above (or similar) for more information about the extension itself.

Open the 2.8 admin manual's “Enabling LDAP authentication” page and review the existing configuration instructions. Add the requested IMPORTANT note with the php-ldap prerequisite, the Ubuntu install command, and an extension reference; done means the requirement is visible before users begin LDAP setup.

Written by the indexing model from the issue text.