feat: Lehrer-Online - optional '.env'-setting to choose which sub-portals should be crawled

[Criamos]

This PR implements a feature-request from 2023-08-10 by Team4 (Romy):

• a (completely optional) .env-setting for lehreronline_spider which allows us to control which "Themenportale" should be skipped during a crawl
  • currently possible settings:
    • "hwm" ignores all URLs from handwerk-macht-schule.de
    • "pubertaet" ignores all URLs from "Themenportal Pubertät"
  • default behavior: crawl all URLs as they are delivered by the Lehrer-Online API
• docs: update .env.example with examples for new, strictly optional setting