Docs: Initial documentation structure using Sphinx (#5355)

This PR is the initial documentation structure including the basic information about the project and Sphinx configuration.

####CONTENTS:
- Getting Started
- Installing
- Boards
- Libraries
- Library Builder
- Tutorials
- ESP-IDF as Component
- OTA Web Update
- makeEspArduino
- Troubleshooting
- Contributing

This PR also changes the README.md.

.gitignore

@@ -18,3 +18,6 @@ __vm/
 .vscode/
 platform.sloeber.txt
 boards.sloeber.txt
+
+# Ignore docs build (Sphinx)
+docs/build

README.md

@@ -1,18 +1,25 @@
 # Arduino core for the ESP32
-[![Build Status](https://travis-ci.org/espressif/arduino-esp32.svg?branch=master)](https://travis-ci.org/espressif/arduino-esp32) ![](https://github.com/espressif/arduino-esp32/workflows/ESP32%20Arduino%20CI/badge.svg)
+
+[![Build Status](https://travis-ci.org/espressif/arduino-esp32.svg?branch=master)](https://travis-ci.org/espressif/arduino-esp32) ![](https://github.com/espressif/arduino-esp32/workflows/ESP32%20Arduino%20CI/badge.svg) [![Documentation Status](https://readthedocs.com/projects/espressif-arduino-esp32/badge/?version=latest&token=b6c5d887f5e0bcbf37a92cf3bf9f93f9e77b9d805c0f708cf128ac04a2228299)](https://docs.espressif.com/projects/arduino-esp32/en/latest/?badge=latest)
 
 ### Need help or have a question? Join the chat at [![https://gitter.im/espressif/arduino-esp32](https://badges.gitter.im/espressif/arduino-esp32.svg)](https://gitter.im/espressif/arduino-esp32?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 ## Contents
+
 - [ESP32-S2 and ESP32-C3 Support](#esp32-s2-and-esp32-c3-support)
 - [Development Status](#development-status)
-- [Installation Instructions](#installation-instructions)
 - [Decoding Exceptions](#decoding-exceptions)
 - [Issue/Bug report template](#issuebug-report-template)
-- [ESP32Dev Board PINMAP](#esp32dev-board-pinmap)
 
 ### ESP32-S2 and ESP32-C3 Support
-If you want to test ESP32-S2 and/or ESP32-C3 through the board manager, please use the development release link: `https://raw.githubusercontent.com/espressif/arduino-esp32/gh-pages/package_esp32_dev_index.json` and install the latest 2.0.0 version.
+
+If you want to test ESP32-S2 and/or ESP32-C3 through the board manager, please use the development release link:
+
+```
+https://raw.githubusercontent.com/espressif/arduino-esp32/gh-pages/package_esp32_dev_index.json
+```
+
+Now you can install the latest 2.0.0 version from the boards manager.
 
 ### Development Status
 
@@ -20,20 +27,16 @@ Latest Stable Release  [![Release Version](https://img.shields.io/github/release
 
 Latest Development Release  [![Release Version](https://img.shields.io/github/release/espressif/arduino-esp32/all.svg?style=plastic)](https://github.com/espressif/arduino-esp32/releases/latest/) [![Release Date](https://img.shields.io/github/release-date-pre/espressif/arduino-esp32.svg?style=plastic)](https://github.com/espressif/arduino-esp32/releases/latest/) [![Downloads](https://img.shields.io/github/downloads-pre/espressif/arduino-esp32/latest/total.svg?style=plastic)](https://github.com/espressif/arduino-esp32/releases/latest/)
 
+### Documentation
 
-### Installation Instructions
-- Using Arduino IDE Boards Manager (preferred)
-  + [Instructions for Boards Manager](docs/arduino-ide/boards_manager.md)
-- Using Arduino IDE with the development repository
-  + [Instructions for Windows](docs/arduino-ide/windows.md)
-  + [Instructions for Mac](docs/arduino-ide/mac.md)
-  + [Instructions for Debian/Ubuntu Linux](docs/arduino-ide/debian_ubuntu.md)
-  + [Instructions for Fedora](docs/arduino-ide/fedora.md)
-  + [Instructions for openSUSE](docs/arduino-ide/opensuse.md)
-- [Using PlatformIO](docs/platformio.md)
-- [Building with make](docs/make.md)
-- [Using as ESP-IDF component](docs/esp-idf_component.md)
-- [Using OTAWebUpdater](docs/OTAWebUpdate/OTAWebUpdate.md)
+You can use [Arduino-ESP32 Online Documentation](https://docs.espressif.com/projects/arduino-esp32/en/docs-initial_version/index.html) to get all information about this project.