feat(translations): Add search placeholder translations

[asierrayk]

Prerequisites

• My Pull Request is filled against the next branch
• All commits in my Pull Request conform to Elegant Git commit Guidelines

Recommended Steps

• My patch adds a new feature, therefore I have also added a help article about it
• My patch changes Elegant behavior, therefore I have updated the help article to reflect this change
• My commits are signed

Description

Tranlations for the search placeholders, adding the following code to the pelicanconf.py:

SEARCH_PLACEHOLDER = 'Buscar'
SEARCH_TAG_PLACEHOLDER = 'Buscar etiqueta'