Doc: Replace plain URLs to OpenGFX/SFX/MSX with Markdown links.

[PeterN]

Motivation / Problem

Plain URLs in README.md are not clickable in the new in-game document viewer.

Description

Replace plain URLs with markdown links, in README.md. There are other links which shouldn't be converted, and probably links in other files that could be.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker