Update docs for 0.7.0

[rkalis]

• Update Covenants guide
• Update example covenants
• Add multiplication operator
• Update 'global covenant variables' section
• Update ANTLR grammar
• Update references to int size
• Add release notes & migration notes
• Remove SLP Mint contracts from showcase (too outdated)
• Update CLI usage docs & other small changes

[rkalis]

These changes are live on https://next.cashscript.org/

[emergent-reasons]

This is great and comprehensive. Hope the nits and questions help a little.