docs-#1020: add nuxt img alt prop definition

[Baroshem]

🔗 Linked issue

Resolves #1020

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This PR adds a note in the documentation about the alt prop that NuxtImg accepts while it is not documented.

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.87%. Comparing base (06f7f77) to head (a6404b6).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1628      +/-   ##
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+ Coverage   62.83%   62.87%   +0.03%     
==========================================
  Files          79       79              
  Lines        3536     3539       +3     
  Branches      411      412       +1     
==========================================
+ Hits         2222     2225       +3     
  Misses       1285     1285              
  Partials       29       29              

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[danielroe]

<NuxtImg> accepts all the possible image props, including alt:

image/src/runtime/components/_base.ts

Lines 31 to 52 in 92a6eee

	width: { type: [String, Number], required: false },
	height: { type: [String, Number], required: false },
	alt: { type: String, required: false },
	referrerpolicy: { type: String as unknown as () => ImgHTMLAttributes['referrerpolicy'], required: false },
	usemap: { type: String, required: false },
	longdesc: { type: String, required: false },
	ismap: { type: Boolean, required: false },
	loading: {
	type: String as () => 'lazy' | 'eager',
	required: false,
	validator: (val: any) => ['lazy', 'eager'].includes(val),
	},
	crossorigin: {
	type: [Boolean, String] as unknown as () => 'anonymous' | 'use-credentials' | boolean,
	required: false,
	validator: (val: any) => ['anonymous', 'use-credentials', '', true, false].includes(val),
	},
	decoding: {
	type: String as () => 'async' | 'auto' | 'sync',
	required: false,
	validator: (val: any) => ['async', 'auto', 'sync'].includes(val),
	},

I think if we mention it here we should note that <NuxtImg> does not apply any special handling for alt but reiterate how important it is to provide it, and include a link to MDN.

[Baroshem]

Fixed @danielroe

Could you check if now it looks good? :)

Should I also write the importance of setting this attribute?