Make links to tutorial and virtual-environment relative in index.md

[gkeuccsr]

Otherwise running a custom build refers to a official published versions and not the ones build (in PRs, for ex.)

[gkeuccsr]

Yikes, readme.md must be equal to docs index.md :(

[YuriiMotov]

Yikes, readme.md must be equal to docs index.md :(

Yes, I think this is the reason why we have absolute links in index.md. readme.md is generated automatically from index.md. And links in the readme should be absolute.

I suggest we close this PR.

@gkeuccsr, thanks for your interest and efforts!