Add breadcrumbs to the bottom of each manual page

Summary:

Fixes #74

A little hacky, but got breadcrumbs to the bottom. But this will do for now.

Test Plan:

Reviewers:

CC:

Task ID: #

Blame Rev:
Change the commit template to be less verbose. Add to .gitignore
Add a nice, canonical async example - coalesced fetching

Summary: Even while we are still finalizing our async foundation, let's add a useful example that can be used now.

Fixes #97

Test Plan: Docs validate and render

Reviewers: ptarjan, jan
Change "around" to "with" in fun() section of Unsupported

Fixes #53
Merge all of trait requirements info into one page.

Fixes #60
Fix "check type" to "type check" in Why Nullable?

Fixes #61
Fix typo in Hack intro (distingusih -> distinguish)

Fixes #63
Remove safe mode and everything that goes with it.

Fixes #64
Remove slice from Vector and ImmVector. This is obsolete and deprecated. It will be replaced by skip() soon

Fixes #
Correct and enhance the description of Set|ImmSet<Tv>::toArray()

Fixes #104