Adapt language variables names to current naming convention and update variables to cover new strings

[vitaminace33]

On 822e3c0e5598c24b7c450628d9bd68d9f6fd58e2: Variables names in languages.yaml did not follow the established convention.

On afcde3d7d121822e5184a9de9a8e18da155c8cde: Add pt/pt-BR to languages.yaml. Credits to @luizvaz.

On 0e095e57ca66b5583691dcf8374f7964149c26ca: Update language variables to cover new strings.

• GITHUB.BUTTON, and GITHUB.NOTE replace GITHUB_EDIT and GITHUB_NOTE.
• GITHUB.TOOLTIP, NAV_PREV, and NAV_NEXT are new and cover tooltips.
• SEARCH.TEXT replaces SEARCH_DOCUMENTATION.
• SEARCH.ADVC is new.
• POWERED_BY_GRAV replaces BUILT_WITH_GRAV

The new text from GITHUB.NOTE has been added to the other languages to enforce contributions.

Fullty translated to Spanish to provide proper testing.