Fix broken installation documentation link in README.txt

[sadiapeerzada]

The link to J5.x:Installing_Joomla in section 8 of README.txt returns a 404.

The installation documentation has been migrated to manual.joomla.org.

This PR updates the link to the current installation guide:
https://manual.joomla.org/docs/get-started/installation/

[brianteeman]

that changes it to a link for how to install joomla locally. If you look at the next line in the readme then its already there. This link is supposed to be a beginner level install and originaly was https://docs.joomla.org/J4.x:Installing_Joomla - there was never a j5 version of that page as nothing had changed. ;)

The correct link therefore would be https://guide.joomla.org/user-manual/getting-started/getting-started-installing-joomla

[LadySolveig]

Yes you are right. I used the sentence that followed as a guide, but didn't read any further. It was only a suggestion anyway, as the suggested link was definitely broken too.

[charvimehradu]

Hi @sadiapeerzada,
Thankyou for your contribution. While reviewing this, I also noticed that there are several other links in the same file that currently lead to missing or incomplete documentation pages.

Because of that, it might be better to first ensure that the corresponding pages are properly available on the Joomla Documentation website and then update the links here accordingly. This would help avoid pointing to incomplete documentation.