Remove mentions of blueprints from our latest documentation

[ricab]

Description of documentation issue or suggestion
Before we had two versions of our documentation (stable and latest), we decided to keep blueprint docs around, tagged as outdated, for another feature release. The goal was to accommodate people using old versions of Multipass and ease the transition away from the obsolete feature. Documentation of blueprints is now obsolete and should be removed now from the latest version.

Proposal to fix issue or enact suggestion
Remove documentation and mentions of blueprints throughout our docs, from the development version.

Additional context
https://documentation.ubuntu.com/multipass/latest/search/?q=blueprints&check_keywords=yes&area=default