docs: Add explanation of Vite and how it's used in MAAS UI MAASENG-3596

[ndv99]

Done

• Added explanation of Vite to the "React" section of MAASUI.md
  • Explained native ES modules and HMR
  • Explained lazy-loading and manual chunks

QA steps

• [ ] Read through the new section and make sure it makes sense
• [ ] Leave a comment if you feel there is anything missing from this section

Fixes

Fixes MAASENG-3596

[webteam-app]

Demo

Jenkins

demos.haus

[tmerten]

:partying_face: :book: