Closes #767 - Fix Broken documentation links.

[fluffyemily]

Addressing comments and issues raised in #767

• Fix broken API documentation links.
• Unify SDKs and Mentat under same version number.
• Reorganize API documentation under version number.
• Add latest symlink to point to the latest API documentation version.
• Update .gitignore to ignore documentation related directories and files.
• Add README to /docs to include local build instructions.
• Update API documentation.

[ncalexan]

This isn't working locally because the symlink you've created is absolute and references your local hard disk layout. There is a way to do symlinks in git, at least away from Windows, which should work with a relative link.

But even if I fix the symlink locally, the front page hyperlinks don't seem correct.

However, I was able to view all three documentation subsections, which was really helpful. I certainly see that our existing rustdoc isn't really helping anyone, so we'll have to work to improve that.

[fluffyemily]

So it turns out that you have to create the symlink inside the directory where the link it so be created in order for git to understand the link, so ln -s docs/0.7 docs/latest won't work but cd docs && ln -s 0.7 latest will.

Also, I stupidly didn't check in the correct editing of the link in index.md. Sorry.

[ncalexan]

This looks much better locally!