docs: add signal form example + copy for matInput

[msmallest]

Part of this discussion: #32072 (comment)

• Adds an example for email input errors, like the reactive form equivalent
• Adds the general Use with Angular Forms section with all 3 types
• Mentions how to implement ErrorStateMatcher with signal forms
• Tweaks example error title/error state matcher copy of for reactive forms to highlight similarities/differences