update Gitpod - use docker image

Working vscode modules on Gitpod:

* cmake configure, compile and provide intellisense

* Microsoft C/C++ tools for intellisense, autocomplete, etc

* use docker image for Gitpod for more control on packages

* enable badges and other convenience tools using Gitpod

.gitpod.dockerfile

@@ -0,0 +1,9 @@
+FROM gitpod/workspace-full
+
+RUN sudo apt-get update \
+ && sudo apt-get install -y \
+    doxygen \
+    graphviz \
+    ninja-build \
+ && pip install cpplint \
+ && sudo rm -rf /var/lib/apt/lists/*

.gitpod.yml

@@ -1,6 +1,17 @@
-tasks:
-- init: >
-  pip install cpplint
-  cmake -S . -B build -DCMAKE_BUILD_TYPE=Debug -DCMAKE_EXPORT_COMPILE_COMMANDS=ON &&
-  cmake --build build -t all 
-  
+image:
+  file: .gitpod.dockerfile
+
+github:
+  prebuilds:
+    addBadge: true
+    addComment: false
+    addCheck: false
+    master: true
+    pullRequestsFromForks: true
+
+vscode:
+  extensions:
+    - ms-vscode.cpptools@0.28.3:mjRj37VUK0nY2ZeDXzxOJA==
+    - twxs.cmake@0.0.17:9s7m9CWOr6i6NZ7CNNF4kw==
+    - ms-vscode.cmake-tools@1.4.0:eP3hU/MFme+CcSL21Klk1w==
+    - mhutchie.git-graph@1.23.0:TM9ShNmBn94aUJMJusCJlg==

README.md

@@ -1,5 +1,5 @@
 # The Algorithms - C # {#mainpage}
-[![Gitpod Ready-to-Code](https://img.shields.io/badge/Gitpod-Ready--to--Code-blue?logo=gitpod)](https://gitpod.io/#https://github.com/TheAlgorithms/C) 
+[![Gitpod Ready-to-Code](https://img.shields.io/badge/Gitpod-Ready--to--Code-blue?logo=gitpod)](https://gitpod.io/#https://github.com/kvedala/C) 
 [![Gitter chat](https://img.shields.io/badge/Chat-Gitter-ff69b4.svg?label=Chat&logo=gitter&style=flat-square)](https://gitter.im/TheAlgorithms)
 [![contributions welcome](https://img.shields.io/static/v1.svg?label=Contributions&message=Welcome&color=0059b3&style=flat-square)](https://github.com/kvedala/C-Plus-Plus/blob/master/CONTRIBUTING.md) 
 ![GitHub repo size](https://img.shields.io/github/repo-size/kvedala/C-Plus-Plus?color=red&style=flat-square)