chore: fix dead links like /themes

[julien-deramond]

Description

This PR suggests fixing some dead links in several files:

• https://sli.dev/themes/use or https://sli.dev/themes/use.html → https://sli.dev/guide/theme-addon#use-theme
• https://sli.dev/themes/gallery or https://sli.dev/themes/gallery.html → https://sli.dev/resources/theme-gallery

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[KermanX]

Thanks a lot~

[pkg-pr-new]

Open in Stackblitz

create-slidev

pnpm add https://pkg.pr.new/slidevjs/slidev/create-slidev@1886

@slidev/client

pnpm add https://pkg.pr.new/slidevjs/slidev/@slidev/client@1886

create-slidev-theme

pnpm add https://pkg.pr.new/slidevjs/slidev/create-slidev-theme@1886

@slidev/cli

pnpm add https://pkg.pr.new/slidevjs/slidev/@slidev/cli@1886

@slidev/parser

pnpm add https://pkg.pr.new/slidevjs/slidev/@slidev/parser@1886

@slidev/types

pnpm add https://pkg.pr.new/slidevjs/slidev/@slidev/types@1886

commit: b993460