Make the separate page for installation instructions, improve README.md

[oleg-derevenetz]

Changelist:

• All installation instructions for pre-built binaries are moved from README.md to doc/INSTALL.md and made more detailed;
• Fixed links to other markdown files in README.md (they used to be handled incorrectly with GitHub Pages), added link to the build instructions for PS Vita.

You can get the overall impression of changes in GitHub Pages mode here: https://oleg-derevenetz.github.io/fheroes2/

[qarkai]

Suggestion: add APT links for Ubuntu. Maybe in another PR.

[qarkai]

LGTM

[ihhub]

Hi @oleg-derevenetz , I have some idea how to make this even more user friendly. Since many (according to statistics) use our deployed website could we just create a separate MD file for this website which will be the main page and making it more user friendly. We could go even with few pages if needed.

I think @Branikolog had multiple suggestions about it.

Of course it's a bit out of the scope of this pull request and we can move to a Discussion topic if needed.

[oleg-derevenetz]

Hi @ihhub

The proposed INSTALL.md file is already a part of this deployed website, you can see it here:

https://oleg-derevenetz.github.io/fheroes2/

This is how you website will look after the merge of this PR.

[ihhub]

@oleg-derevenetz , thanks a lot for improving the documentation!

[oleg-derevenetz]

Hi @ihhub and @Branikolog if there will be any specific ideas regarding website improvement (or better yet, a some sort of a sketch), let's discuss :) I think, @Branikolog can make a PR (I can help in technical terms, if necessary).

[Branikolog]

Hi, @oleg-derevenetz !
I definitely have some ideas as well a feedback from our community regarding website. We should make it as friendly and clear as possible.
Not sure, that I can make a PR, but I can write here a little bit of my thoughts. Tomorrow, I think.