Fix broken links in documentation

[ckunki]

Eg. to postgres virtual schema

• actual: https://github.com/exasol/postgresql-virtual-schema/blob/main/doc/dialects/postgresql.md
• expected: https://github.com/exasol/postgresql-virtual-schema/blob/main/doc/user_guide/postgresql_user_guide.md

Additionally [Document Files][virtual-schema-common-document-files] appears to be broken.