extracted parts which bikeshed can reuse across the specs

[elf-pavlik]

• shared Terminology section
• shared bibliography (any references currently not available at https://www.specref.org/ )
• mention the potential of reusing in Create WebPushChannel2023 #140
• track improvement of linking Link terms used by channel types to definitions in the protocol #161

[csarven]

Haven't tested the changes (by building) but they look correct and as per agreement.

[elf-pavlik]

I run the build locally for both channel types. Automatic checks for all PRs will be taken care of in #156