Add mnemonics scraper

Add scrapy spiders to scrape parse and load data from:

• https://biolympiads.com/list-of-chemistry-mnemonics/
• https://en.wikipedia.org/wiki/List_of_mnemonics
• https://en.wikipedia.org/wiki/List_of_visual_mnemonics

Data visualization: https://charts.mongodb.com/charts-project-0-xoixf/embed/charts?id=52c84f3f-f267-4ecb-a36f-7f71ea9a24e4