Docs: `README.md` duplicates `README.en.md` and the two will drift

[beyondnetPeru]

This repo ships three README files: README.md, README.en.md and README.es.md. README.md is already a near-copy of the English version and links to it as "detailed documentation" — so English content lives in two files that nobody keeps in sync.

The same layout exists in Shell.ddd and Shell.Factory, while Shell.Bootstrapper has only README.md + README.es.md. So the family is inconsistent on top of being duplicated.

Proposal: README.md becomes the English document and the single source of truth, with a language switcher at the top; README.en.md is deleted; README.es.md stays as the translation. Apply the same layout across all four Shell.* repos.

Acceptance criteria

• README.en.md removed, with any content unique to it merged into README.md.
• Language switcher present in both remaining files, pointing at paths that exist.
• No internal link anywhere in the repo still points to README.en.md.

Where

README.md, README.en.md, README.es.md.