Update documentation about easy-rsa certificate generation

louvre is now a new debian stable (buster) which drags a new easy-rsa version 3. The modus operandi is different.

This means:

• migration? [1]

• the intranet documentation [2] needs to be updated once understood how that works [3] [4]

• [1] https://github.com/OpenVPN/easy-rsa/blob/master/doc/EasyRSA-Upgrade-Notes.md

• [2] https://intranet.softwareheritage.org/wiki/VPN#For_admins

• [3] https://github.com/OpenVPN/easy-rsa/blob/master/README.quickstart.md

• [4] https://github.com/OpenVPN/easy-rsa/blob/master/doc/EasyRSA-Readme.md

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Migrated from T1940 (view on Phabricator)