Update documentation navigation

[AlexB52]

Fixes #818

Problems:

• Looking at the source all basics & advanced pages are available through the documentation navigation except "Template Engine Options".
• Also some locales are unused in the navigation.

Here is a PR that:

• Removes the unused locales
• Moves the "Template Engine Options" pages from basic to advanced folder
• Displays the "Template Engine Options" in the advanced section of the documentation
• Add the relevant Netlify redirects to the template-engine-options page

This is ok if the PR is not accepted. I just wanted to contribute to some extent to the project and create discussion around that "hidden" page.

Considerations

I tried to merge "Templating Language" and "Template Engine Options" into one page but it feels off to appear in the basics section. I decided to move it to the advanced section instead.

What do you think?

[tdreyno]

Thanks!