[doc]: Add other install methods on official webpage

[douyixuan]

Actually, after I compiled the source, there is a brew formula available. It would be easier to install re2c using Homebrew or other package managers.

brew install re2c

[skvadrik]

I don't think such information belongs on the re2c website. It varies from one OS/distro to another, it changes with time, there are often a few different package managers and ways to install a program, and it would be very hard to maintain up to date information on re2c website. Target OS forums / documentation is a much better source of information.

[trofi]

It usually takes quite a bit of effort to mention even most popular repositories (here is a helix example).

WDYT of just mentioning the list of repositories that package re2c? Would it be enough for you to start looking ahow to install it from there?

Example badge: https://repology.org/badge/vertical-allrepos/re2c.svg

[douyixuan]

Thank you for considering this. The packaging status has been extremely helpful.

[skvadrik]

I added Repology links on the main page (just a link) and on the build page (embedded SVG). It won't show on the official website yet as I can't publish some other changes before the next release, but it will be there after release 4.1.