Improve online documentation

[JeanPaulShapo]

Main parts:

• make new section 'Installation'
• move Windows and Linux tutorials to legacy pages

However, there is need to fix some broken links later.

[JeanPaulShapo]

Fixed #386

[JeanPaulShapo]

Hope it fixed #395

[sashafrey]

• Please re-create this pull request against bigartm/stable branch, because it is really useful and we would like to publish it immediately.
• Remove vim:ft=rst at the end of tutorials/python_api.

[JeanPaulShapo]

@sashafrey I wonder why remove vim:ft=rst only in tutorials/python_api :)

[sashafrey]

@JeanPaulShapo I haven't noticed it in other places :)

[JeanPaulShapo]

@sashafrey Ok, I fix it in the next commit.

[sashafrey]

@JeanPaulShapo I've added you to commiters group, so feel free to merge your pull request for documentation. Please, create them against bigartm/stable branch because this is the default branch that show up at http://docs.bigartm.org. I will regularly merge bigartm/stable into bigartm/master.

[sashafrey]

For the code I'd still prefer to review and discuss changes, but for documentation I think its OK to just merge. Documentation is now in such a bad state that any change will improve it :) So I fully trust you on this! :)

[JeanPaulShapo]

@sashafrey Is it OK not only merge 'documentaton' pull requests but also close 'documentation' issues related to these pull requests?

[sashafrey]

@JeanPaulShapo yes, sure!