Projections.md do not mention `allLanguages`

[MMilosz]

Please describe your request
PR DSpace/DSpace#11718 introduced a new projection allLanguages (implemented in AllLanguagesProjection)

This projection should be described in Projections.md

JavaDoc from the AllLanguagesProjection:

/**
 * This projection. When set,
 * no metadata languages filtration will be performed during DSpaceObject conversion.
 * will return all metadata in all languages.
 *
 * @author Mohamed Eskander (mohamed.eskander at 4science.it)
 *
 */

The default projection hides metadata values in languages other than the one requested, unless ?projection=allLanguages is explicitly specified. The typical use cases are in-progress submissions (so that all language variants can be edited) and the full item view (e.g. https://sandbox.dspace.org/items/92aec336-11f4-4653-851e-885fbb07f8e5/full, note the /full at the end)