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.

docs/source/contributing.rst

@@ -0,0 +1,49 @@
+###################
+Contributions Guide
+###################
+
+We welcome contributions to the Arduino ESP32 project!
+
+How to Contribute
+-----------------
+
+Contributions to the Arduino ESP32 (fixing bugs, adding features, adding documentation) are welcome.
+We accept contributions via `Github Pull Requests <https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/about-pull-requests>`_.
+
+Before Contributing
+-------------------
+
+Before sending us a Pull Request, please consider this:
+
+* Is the contribution entirely your own work, or is it already licensed under an LGPL 2.1 compatible Open Source License? If not, we unfortunately cannot accept it.
+
+* Is the code adequately commented for people to understand how it is structured?
+
+* Is there documentation or examples that go with code contributions?
+
+* Are comments and documentation written in clear English, with no spelling or grammar errors?
+
+* Example contributions are also welcome.
+  * If you are contributing by adding a new example, please use the `Arduino style guide`_.
+
+* If the contribution contains multiple commits, are they grouped together into logical changes (one major change per pull request)? Are any commits with names like "fixed typo" `squashed into previous commits <https://eli.thegreenplace.net/2014/02/19/squashing-github-pull-requests-into-a-single-commit/>`_?
+
+If you're unsure about any of these points, please open the Pull Request anyhow and then ask us for feedback.
+
+Pull Request Process
+--------------------
+
+After you open the Pull Request, there will probably be some discussion in the comments' field of the request itself.
+
+Once the Pull Request is ready to merge, it will first be merged into our internal git system for in-house automated testing.
+
+If this process passes, it will be merged onto the public github repository.
+
+Legal Part
+----------
+
+Before a contribution can be accepted, you will need to sign our contributor-agreement. You will be prompted for this automatically as part of the Pull Request process.
+
+
+
+.. _Arduino style guide: https://www.arduino.cc/en/Reference/StyleGuide