Add Spanish formality level to the Translating Gitlab documentation page

Why this MR?

While contributing to translating Gitlab to Spanish in Crowdin I noticed ambiguity in the formality level in various translations in Spanish (i.e., some are formal, some informal). According to the Glossary, the formality level for Spanish should be informal ("you" -> "tú") instead of formal ("you" -> "usted"). Staying this in the formality level section of the Translating Gitlab's documentation page might help contributors to avoid this mistake.

Examples of this formality ambiguity

These examples are extracted from CrowdIn translations in GitLab.pot:

Formal	Informal

Context about Spanish formality levels

Next is information about the formality levels according to the "Real Academia de la Lengua", the institution that regulates and informs about the Spanish conventions (those references are written in Spanish):

• Usted (Formal)
• Tú (Informal)
• Vos (Informal in some countries)

What this MR does?

This MR adds the formality level for Spanish according to the convention shown in the Glossary in the Translating Gitlab's documentation page, so it's easy to understand for the contributor the way the translations should be made.

MR acceptance checklist

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