docs: enhance README with instructions and helpers section

[cedric07]

Pour faire suite à cette card : https://www.notion.so/beapi/Documentation-technique-BFF-et-BBT-2140865393b98043a207e12da097b791?v=28b1760024c04863922bd6a44b0ee494&source=copy_link

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Note

Low Risk
Low risk: documentation-only changes with no runtime or build logic modifications; main risk is minor confusion if examples drift from actual helper behavior.

Overview
Expands README.md with a new Formatting helpers section documenting how to use the theme’s namespaced helper functions (Image, Link, Share, Term, Text, and SVG sprites), including example calls, expected HTML output, and related filters.

Clarifies Composer PSR-4 autoload mapping for inc/, updates the dev workflow section to explicitly document yarn start/yarn build plus new lint commands (yarn lint:css, yarn lint:js, yarn lint), and adds an illustrative example of merged bff_editor_custom_settings output.

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