Add HAProxy helper docs

[nico-stefani]

Description

This PR closes #7232. Add documentation for the HAProxy helper feature. This includes:

• Reference for the new ossec.conf configurations
• New section about how to install and configure HAProxy
• Removed references to NGINX
• New section about how to enable the HAProxy Helper

Checks

Docs building

• Compiles without warnings.

Code formatting and web optimization

• Uses three spaces indentation.
• Adds or updates meta descriptions accordingly.
• Updates the redirects.js script if necessary (check this guide).

Writing style

• Uses present tense, active voice, and semi-formal registry.
• Uses short, simple sentences.
• Uses bold for user interface elements, italics for key terms or emphasis, and code font for Bash commands, file names, REST paths, and code.

[nico-stefani]

The diagrams are drafts, the final ones are currently developing by the marketing team.

[nico-stefani]

Updated with the final version of the diagrams.

[GGP1]

LGTM

[Selutario]

This is a good starting point, congratulations! I have mentioned a few things to make it clearer below.

In my opinion (based on the initial requirements of the objective: Add pages about deploying and configuring HAProxy balancer through packages and docker images.) and taking into account that we intend to support only a specific HAProxy version, I would explain step-by-step how to install and configure it.

Regardless of whether you decide to do the latter, keep in mind that this will be announced as one of the highlights of 4.9 and many users will probably be interested in the helper, so it should be easy for them to configure (since broadly speaking and by its nature, it is a feature complex both to understand and use).

Try, from the point of view of a user who sees the Helper for the first time, to configure both HAProxy LB and the Helper following the documentation and using the resources provided (both the official haproxy docker image and the package). I think there are some places where adding additional information could save a lot of users headaches.

[Selutario]

Very good work here @nico-stefani, this improved version is much easier to understand and follow. There are still some incorrect things, I explain them below:

[javimed]

(This review is partial.)

[Selutario]

Suggested change

	# curl -sL https://github.com/haproxytech/dataplaneapi/releases/download/v2.8.X/dataplaneapi_2.8.X_linux_x86_64.tar.gz | tar xz && cp dataplaneapi /usr/local/bin/
	# curl -sL https://github.com/haproxytech/dataplaneapi/releases/download/v2.8.7/dataplaneapi_2.8.7_linux_x86_64.tar.gz | tar xz && cp dataplaneapi /usr/local/bin/

One last comment on my side: are these Xs intentional? If we don't want to include the patch in case they update it (which seems fine to me), we should at least tell the user where they can find the latest one:

• https://github.com/haproxytech/dataplaneapi/releases/

Maybe something like

1. Download the binary file for the installed HAProxy version. You can find the available versions [here](https://github.com/haproxytech/dataplaneapi/releases/). 

[nico-stefani]

Fixed in 9edc180

[Selutario]

Nice job @nico-stefani!