Add accessibilite documentation: pa11y for testing

accessibility.md

+# Testing with pa11y
+
+This [resource](https://bitsofco.de/pa11y/) is a good start to create your own testing pipeline.
+
+
+
+## Quick and simple overview
+```
+npm install -g pa11y
+```
+```
+pa11y https://data.beta.grandlyon.com/fr/accueil
+```
+
+## Display only critical errors
+```
+pa11y https://data.beta.grandlyon.com/fr/accueil --ignore "warning;notice"
+```
+
+## Using the pa11y-ci
+```
+npm install -g pa11y-ci
+```
+
+Then create a **.pa11y1** json file.
+```
+{
+  "urls": [ 
+    "https://data.beta.grandlyon.com/fr/accueil"
+  ]
+}
+```
+To run the report on multiple URLs:
+```
+pay11-ci
+```
+
+It is also possible to save the report as a json file:
+```
+pay11-ci --json > report.json
+```
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