Enhance section about searchindex rebuild.

[YetAnotherGerrit]

Currently, our documentation about rebuilding the search index is very basic:

https://docs.zammad.org/en/latest/install/elasticsearch.html

There are several things that can be improved:

1. Make it linkable

When a rebuild is needed, e.g. because of an update, it should be easy to send/place a link on how to rebuild the search index. For example, for support, when sending e-mails or in the release notes.

2. Include different deployment methods

Currently, the documentation of the rebuild focuses on the package installation. From source and docker is missing.

3. Enrich information

We could mention that a rebuild can be done, while people work on the instance, but the search results will be incomplete while the reindex is still running. We could also mention, that depending on the size of the database, it can take hours and even days, if only one core is used.

[ralf401]

Link for rebuild of ES searchindex will look like this:

https://docs.zammad.org/en/latest/install/elasticsearch.html#es-rebuild-searchindex