Add Warning for Ubuntu LTS installation instructions

[beny-dl]

Description:

This PR refines the installation section of the mlpack Quickstart guide to better serve Ubuntu LTS users, who may face issues due to outdated libmlpack-dev versions in their repositories (currently version 3.4.2). The lack of the mlpack.hpp header in these versions can hinder new users' ability to run mlpack successfully.

Changes:

• Included a note for Ubuntu LTS users about the older libmlpack-dev version.
• Recommended building mlpack from source or using Docker for accessing the latest version.

Appreciate your review and feedback on this documentation update.

[mlpack-bot]

Thanks for opening your first pull request in this repository! Someone will review it when they have a chance. In the mean time, please be sure that you've handled the following things, to make the review process quicker and easier:

• All code should follow the style guide
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• HISTORY.md updated if the changes are big or user-facing
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Thank you again for your contributions! 👍

[conradsnicta]

Suggest to omit mention of Docker, as that can muddle the problem further.

If a novice user is unable to figure that there is a clash between old and new versions of mlpack, I don't think sending them to the Docker path is helpful.

An advanced user will be able to figure out that Docker is an option anyway, by reading the instructions.

[beny-dl]

Sure, that sounds like a good idea. I'm even thinking that all the second part of the paragraph might be unnecessary. But I guess it doesn't hurt to have it.

Anyway, I committed a version with the docker part removed. Thanks for the feedback!

[mlpack-bot]

Second approval provided automatically after 24 hours. 👍

[mlpack-bot]

Hello there! Thanks for your contribution. I see that this is your first contribution to mlpack. If you'd like to add your name to the list of contributors in COPYRIGHT.txt and you haven't already, please feel free to push a change to this PR---or, if it gets merged before you can, feel free to open another PR.

In addition, if you'd like some stickers to put on your laptop, I'd be happy to help get them in the mail for you. Just send an email with your physical mailing address to stickers@mlpack.org, and then one of the mlpack maintainers will put some stickers in an envelope for you. It may take a few weeks to get them, depending on your location. 👍