OAI-PMH: add XSL stylesheet instructions

[Samk13]

❤️ Thank you for your contribution!

Description

OAI-PMH: add XSL stylesheet instructions

example: https://zenodo.org/oai2d

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• I'm aware of the code of conduct.
• I've created logical separate commits and followed the commit message format.
• I've targeted the master branch.
• If this documentation change impacts the current release of InvenioRDM, I will backport it to the production branch following approval or indicate to a maintainer that it should be backported.

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1. You license your contribution under the same terms as the current repository’s license.
2. You agree that you have the right to license your contribution under the current repository’s license.

[tmorrell]

Great addition with the updated link/license