Add Spanish translation

[Jalkhov]

Language: Spanish

Hi, my name is Pedro Torcatt. I request to add the Spanish translation of Flask's documentation.

Initial tasks

• [x] Create an empty repository on GitHub. Name the repository as flask-docs-<lang code>, for example, flask-docs-es.
• [x] Set up your repository based on the translation template repository:

  $ git clone https://github.com/flaskcwg/translation-template.git flask-docs-<lang code>
  $ cd flask-docs-<lang code>
  $ git remote set-url origin <the-Git-URL-to-your-repository>
  $ git remote add upstream https://github.com/pallets/flask.git
  $ git fetch upstream
  $ git merge upstream/2.0.x
  $ git push -u origin main

• [x] Create a virtual environment, then install the requirements:

  $ python -m pip install --upgrade pip setuptools
  $ pip install -r requirements/dev.txt
  $ pip install sphinx-intl
  $ pip install -e .
  $ pre-commit install

• [x] Generate the .pot and .po files (replace <lang code> with your language code):

  $ cd docs
  $ make gettext  # use ".\make.bat gettext" on Windows
  $ sphinx-intl update -p _build/gettext -l <lang code>

• [x] Replace the <LANG> in the README, the language config, and the html_search_language config in docs/conf.py with your language code. Update the .po file examples.
• [x] Setup ReadtheDocs to connect your repository, use flask-<lang code> as the subdomain, set the default version to the main branch, and enable the "single version" mode.
• [x] Translate the README and the pull request template (optional).
• [x] Update the title, and all the <UPDATE THIS> placeholders in README.md and docs/conf.py. You can update the README to add more format or translate tips for your language.
• [x] Update the Language-Team value in all .po files.
• [x] Translate html_title and project links in the docs/conf.py file.
• [x] Translate the docs/index file (you can make a localized logo for the index page).
• [x] Translate the docs/foreword file.
• [x] Translate the docs/advanced-foreword file.
• [x] Translate the docs/installation file.
• [x] Translate the docs/quickstart file.
• [x] Translate the "Tutorial" part of the docs.
• [x] Switch the repository into the flaskcwg organization.

Public

Tasks after the repository is switched into flaskcwg:

• [x] Add a new language entry to the README.
• [x] Add a new language entry to translations page.

Maintenance

Tasks when the whole documentation is translated:

• [x] Create a 2.0.x branch, set it as default version on ReadtheDocs.
• [x] Rewrite the contributing guide in the README of the translation repository, encourage people to fetch upstream and update docs (see details below).
• [x] Rename the section "Translation To-do List" to "Translators". You can also add a "Reviewers" section.
• [ ] Switch the docs into http://flask.palletsprojects.com/.

You should sync documentation updates from Flask periodically:

$ git fetch upstream
$ git merge upstream/2.0.x
$ cd docs
$ make gettext  # use ".\make.bat gettext" on Windows
$ sphinx-intl update -p _build/gettext

Then update the translation for new changes.