new discovery and error page based on bootstrap-itali v2.6.1

[MdreW]

Fix #80

• added a new discovery page based on last version of bootstrap italia
• added a new error page based on last version of bootstrap italia
• removed unused files
• reorganized static directory

[peppelinux]

@MdreW this PR was merged in this https://github.com/italia/Satosa-Saml2Spid/pull/94#pullrequestreview-1751549225

we look forward for doing some test on the eudi branch before tagging a new release with the OpenID4VP Wallet RP disabled by default