docs: add new external oauth client (#6906)

* docs(external-oauth-client): add new one

* docs(external-oauth): chnage link

Author: njfamirm

website/content/docs/external-oauth-clients.md

@@ -20,5 +20,6 @@ If you would like to facilitate your own OAuth authentication rather than use Ne
 | [@deepbass](https://github.com/deepbass)                     | GitHub, GitHub Enterprise         | Node.js Azure Functions           | [Repo](https://github.com/deepbass/serverless-cms-azure), [Blog](https://www.danielbass.dev/building-a-serverless-cms-on-azure-with-netlify-cms-and-gatsby/) |
 | [@adrian-ub](https://github.com/adrian-ub)                   | GitHub, GitLab                    | TypeScript                        | [Repo](https://github.com/ublabs/netlify-cms-oauth)                                                                                                          |
 | [@hatappo](https://github.com/hatappo)                       | GitHub                            | ClojureScript, Firebase Functions | [Repo](https://github.com/hatappo/netlifycms-oauth-server)                                                                                                   |
+| [@njfamirm](https://github.com/njfamirm)                       | GitHub                            | Node.js, Typescript | [Repo](https://github.com/njfamirm/decap-cms-github-backend), [Blog](https://www.njfamirm.ir/en/blog/git-separate/)                                                                                                   |
 
 Check each project's documentation for instructions on installation and usage.