Add code_style, developer_guide and fix CONTRIBUTING.md (#29210)

* Remove reference to matrix-js-sdk in code_style.md

* Absorb exisiting documentation from matrix-react-sdk

* Crete a separate developer guide

* Remove sign-off from CONTRIBUTING.md

Since sign-off is irrelevant to element-web repo with the introduction
of CLA.

* Link to the new docs in README

* Elaborate on the rule

* Fix lint

CONTRIBUTING.md

@@ -189,89 +189,6 @@ give away to contributors - if you feel that Matrix-branded apparel is missing
 from your life, please mail us your shipping address to matrix at matrix.org
 and we'll try to fix it :)
 
-## Sign off
-
-In order to have a concrete record that your contribution is intentional
-and you agree to license it under the same terms as the project's license, we've
-adopted the same lightweight approach that the Linux Kernel
-(https://www.kernel.org/doc/html/latest/process/submitting-patches.html), Docker
-(https://github.com/docker/docker/blob/master/CONTRIBUTING.md), and many other
-projects use: the DCO (Developer Certificate of Origin:
-http://developercertificate.org/). This is a simple declaration that you wrote
-the contribution or otherwise have the right to contribute it to Matrix:
-
-```
-Developer Certificate of Origin
-Version 1.1
-
-Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
-660 York Street, Suite 102,
-San Francisco, CA 94110 USA
-
-Everyone is permitted to copy and distribute verbatim copies of this
-license document, but changing it is not allowed.
-
-Developer's Certificate of Origin 1.1
-
-By making a contribution to this project, I certify that:
-
-(a) The contribution was created in whole or in part by me and I
-    have the right to submit it under the open source license
-    indicated in the file; or
-
-(b) The contribution is based upon previous work that, to the best
-    of my knowledge, is covered under an appropriate open source
-    license and I have the right under that license to submit that
-    work with modifications, whether created in whole or in part
-    by me, under the same open source license (unless I am
-    permitted to submit under a different license), as indicated
-    in the file; or
-
-(c) The contribution was provided directly to me by some other
-    person who certified (a), (b) or (c) and I have not modified
-    it.
-
-(d) I understand and agree that this project and the contribution
-    are public and that a record of the contribution (including all
-    personal information I submit with it, including my sign-off) is
-    maintained indefinitely and may be redistributed consistent with
-    this project or the open source license(s) involved.
-```
-
-If you agree to this for your contribution, then all that's needed is to
-include the line in your commit or pull request comment:
-
-```
-Signed-off-by: Your Name <your@email.example.org>
-```
-
-We accept contributions under a legally identifiable name, such as your name on
-government documentation or common-law names (names claimed by legitimate usage
-or repute). Unfortunately, we cannot accept anonymous contributions at this
-time.
-
-Git allows you to add this signoff automatically when using the `-s` flag to
-`git commit`, which uses the name and email set in your `user.name` and
-`user.email` git configs.
-
-If you forgot to sign off your commits before making your pull request and are
-on Git 2.17+ you can mass signoff using rebase:
-
-```
-git rebase --signoff origin/develop
-```
-
-## Private sign off
-
-If you would like to provide your legal name privately to the Matrix.org
-Foundation (instead of in a public commit or comment), you can do so by emailing
-your legal name and a link to the pull request to dco@matrix.org. It helps to
-include "sign off" or similar in the subject line. You will then be instructed
-further.
-
-Once private sign off is complete, doing so for future contributions will not
-be required.
-
 # Review expectations
 
 See https://github.com/element-hq/element-meta/wiki/Review-process