Add initial panels for Weblate

[mafesan]

This PR includes:

• The JSON file for the exported Weblate Overview panel.
• The JSON file for the exported Weblate Demographics panel.
• The JSON file with the Weblate index pattern.
• Documentation about these panels.

Note: the documentation for "Weblate Overview" panel includes two links that right now don't exist, but they will have to be created soon:

• Link to Weblate schema CSV (https://github.com/chaoss/grimoirelab-elk/blob/master/schema/weblate.csv). Edit: this link should work after creating the schema file for this index pattern in PR https://github.com/chaoss/grimoirelab-elk/pull/944.
• Link to "How to deploy" section on Mordred (https://github.com/chaoss/grimoirelab-sirmordred#weblate-). Edit: this should work after accepting PR https://github.com/chaoss/grimoirelab-sirmordred/pull/489.

[mafesan]

Note:

• Schema file for this index pattern updated in PR https://github.com/chaoss/grimoirelab-elk/pull/944.
• "How to deploy" section on Mordred updated in PR https://github.com/chaoss/grimoirelab-sirmordred/pull/489.