Add Table of Contents to pages for viewing via github

_Summary: There's a really easy way to do this, and keep it uptodate - this ticket is to explore whether you would accept a pull-request to implement it..._

## Motivation

Having seen the Table of Contents links on some docs on github, I've found them to be really useful in getting any overview of the topics covered, and then also for really easy navigation.

Examples are:

* https://github.com/catchorg/Catch2/blob/master/docs/assertions.md#top
* https://github.com/approvals/ApprovalTests.cpp/blob/master/doc/Contributing.md#top

I keep coming back to https://github.com/lefticus/cppbestpractices/blob/master/02-Use_the_Tools_Available.md to look for different types of information, and it would save a lot of time and scrolling if users could jump to the section of interest.

## Alternatives considered

Searching through the existing issues, I saw #9 which said the solution was to use gitpages,

I downloaded the gitpages PDF, and it doesn't have tables of contents for sections.

And its "Read" feature gives "This site can't be reached" - ERR_CONNECTION_TIMED_OUT - https://lefticus.gitbooks.io/cpp-best-practices/content/

Given how quickly github.com loads Markdown pages, coupled with the convenience of being able to see page histories there and even possibly suggest improvements to the docs, I feel that it's preferable to add Table of Contents to each of the live pages on github.

## Possible implementation

For ApprovalTests.cpp, we are using https://github.com/thlorenz/doctoc

It's easy to install:

`npm install -g doctoc`

Then we run it with this on Windows:

`doctoc --title **Contents** .`

or this on Unix:

`doctoc --title '**Contents**' .`

A minor enhancement is to move the generated ToC to after the level-1 heading, so that only level-2 and above headings are included in the ToC.

## Consequences

The nice this is that this doesn't require all `cppbestpractices` contributors to have this tool.

Many edits won't change the ToC, and for those that do, `doctoc` could be run later by someone who has the tool. (I'd be happy to do that, but it would be better done by someone who has commit-permission on the repo)