OAI-PMH: add XSL stylesheet instructions

docs/reference/oai_pmh.md

@@ -63,7 +63,7 @@ The OpenAIRE OAI sets is included by default in InvenioRDM.
 The prefix that will be applied to the generated OAI-PMH ids. Should be set to the domain of the repository (f.e. `inveniordm.docs.cern.ch`):
 
 ```python
-OAISERVER_ID_PREFIX = 'inveniordm.docs.cern.ch':
+OAISERVER_ID_PREFIX = 'inveniordm.docs.cern.ch'
 ```
 
 #### Admin Emails
@@ -75,3 +75,13 @@ OAISERVER_ADMIN_EMAILS = [
     'info@inveniosoftware.org',
 ]
 ```
+
+## XSL Stylesheet
+
+To enhance the presentation of OAI 2.0 results for users navigating to the OAI Server, you can define an XSL Stylesheet to transform XML into HTML. You have the option to use a static XSL file or fetch it from a remote server.
+
+To use a local XSL Stylesheet, you can download a pre-defined XSL Stylesheet for OAI 2.0 from the [EPrints repository](https://raw.githubusercontent.com/eprints/eprints/3.3/lib/static/oai2.xsl) (GPLv3 licensed). Place the file in a `static` folder within your instance, run `invenio-cli assets build` to build the assets, and set the relative URL in the `invenio.cfg`:
+
+```python
+OAISERVER_XSL_URL = '/static/oai2.xsl'
+```