doc: move wiki content to docsify page

[skaldarnar]

Contains

Merges the content of the Terasology wiki (from commit e4d4b10424f24eed6583ea0e998da8aa32a27a3f) into the docs/ folder and set up a docsify page for it.

How to test

The page can be previewed locally by starting a simple HTTP server from the docs/ folder, for instance by running

python -m http.server 3000

Outstanding before merging

I think this is mostly functional by now. The wiki content itself is far from perfect, and I already removed a couple of dead links.
However, I think this is a task for follow-up PRs.

• Review to confirm this is in a somewhat usable state
• before or after merging: configure Github to serve Github pages for the engine repo

The following tasks should be performed after merging this:

• (after merge) delete/disable the wiki
• (after merge) check for links to the wiki and correct them
• (optional) merge the content from docs-pre-merge/ into docs, but probably works better in a follow-up PR
• convert or delete .mediawiki pages

[jdrueckert]

Generally looks good, except for

• images and links in included README.md are broken (might be due to the move of docs/images?)
• page titles are not shown except if they're explicitly included in the page content (probably due to wiki magic taking care of that before)

[BenjaminAmos]

I'm getting an error trying to clone this with git (on Windows):
error: invalid path 'docs/Teracon-2017:-Slides-about-multiplayer.md'

If we keep files with : in their names in the repository, then Windows users will always fail to clone it.

[skaldarnar]

page titles are not shown except if they're explicitly included in the page content (probably due to wiki magic taking care of that before)

@jdrueckert This should be addressed by 2a7cb52

[…] files with : in their names in the repository […]

@BenjaminAmos Addressed in 2789867

[skaldarnar]

images and links in included README.md are broken (might be due to the move of docs/images?)

@jdrueckert Without touching the main README in this PR, I instead made the Home page of the wiki the landing page in fae7b47, so at least no broken images/links.

[jdrueckert]

I think this is good enough for the initial migration 👍