Improve the BP Codex chapters organization

[imath]

I've been analyzing the way the codex is organized today. I've updated BP Codex Table of Contents and realized we have a bunch of issues :

• Some parent pages where children are not displayed are useless until they got their children back 😁 (see ​here, ​here and ​there...)
• Some important documentation does not have a parent page which make it difficult to maintain, eg: ​https://codex.buddypress.org/emails/
• Why keeping ​https://codex.buddypress.org/legacy/ and child pages if users can't reach it from the codex navigation ?
• well there's some work :)

You'll find the ​updated TOC attached to this ticket, we'll be able to use it to carry on updating/improving the codex.

NB: We shouldn't use our own tools to host BuddyPress related material, I couldn't do anything with the current ​table of contents and had to restart from scratch.

[imath]

Below is my suggestion about chapters organization simplification :

• Get Started
• Components & Features
• Advanced customizations > ​https://developer.buddypress.org
• Participate and contribute > Get Involved

I've been testing the WordPress HelpHub plugins and theme.

[espellcaste]

Closing as per #9193