Fix visual bugs, improve documentation and add Postgres Harvester

[mjanez]

This pull request includes several updates to the README.md file, enhancements to the ckanext-schemingdcat extension, and improvements to the CSS and translation files. The most important changes include updates to the documentation, new features for custom harvesters, and CSS modifications for better text handling.

Documentation Updates:

• Added details about the requirement for ckanext-harvest for custom harvesters and provided a link to the improved version.
• Included a new section on SQL Harvester and a comprehensive guide on schema generation and field mapping structure.

New Features:

• Introduced the schemingdcat_postgres_harvester plugin for local databases.

CSS Improvements:

• Added CSS classes truncate-text and truncate-link to handle text overflow and ensure better display of long text and links.

Translation Files:

• Updated translation files to include new messages related to harvest source management.

Code Refactoring:

• Replaced toolkit with p.toolkit in ckanext/schemingdcat/logic/auth/ckan.py for better readability and consistency.