If you want to…Then read…
Just get started!Getting Involved, which explains various ways of getting started with Matrix

or

the FAQ, where we try to answer all your questions relating to Matrix
Get started with the CS API using … 
… JavaScriptUsage of the matrix-js-sdk
… PythonMatrix Python SDK documentation
Understand Application ServicesThe Application services guide introduces them and explains what they can be used for
See an example of a simple bottiny-matrix-bot source code
Create a complete working chatbotUsing Matrix to make Chatbot software from the 1960s available in 2018 goes from the basics, brings in matrix-bot-sdk, and ends with deployment on a Raspberry Pi
Work on Bridges to MatrixTypes of Bridging should be read by all bridge developers to ensure everyone has the same mental map of terminology when implementing bridges
Migrate from the old v1 CS APIthe v1 migration guide which justs lists the changes from v1 to r0
Use E2E in a client applicationThe End-to-end Encryption Implementation Guide is intended for client developers who wish to add support for end-to-end encryption to their clients
Install SynapseInstalling Synapse Guide
… using Dockerthe Synapse Docker README
Understand Moderation in MatrixModeration in Matrix

Introduction for Users

Installing Synapse

To install Synapse, we recommend taking a look at the Installing Synapse guide page. Other articles are listed below.

Synapse Maintenance

ArticleAuthorLanguage
Compressing Synapse databaseVictor Berger / LevansEnglish
How to monitor Synapse metrics using PrometheusSynapse TeamEnglish
Using PostgresSynapse TeamEnglish
How to enable VoIP relaying on your Home Server with TURNSynapse TeamEnglish
Setting up FederationSynapse TeamEnglish
Use of Server NoticesSynapse TeamEnglish
Use of OpenTracingSynapse TeamEnglish
Scaling synapse via workersSynapse TeamEnglish
Boosting matrix/synapse by using workers Mark NowiaszEnglish
There are many other howtos and documents in the Synapse repo.

Understanding End-to-End Encryption

ArticleAuthorLanguage
An introduction to end-to-end encryption in Matrix and RiotHubert ChathiEnglish

Translations

ArticleAuthorLanguage
How to translate Riot-WebRiot-Web TeamEnglish

Video Guides