feat: Add sitemap + structured data for SEO

[rkettelerij]

Description

Search Engine Optimization (SEO) by:

• adding a sitemap.xml
• adding a robots.txt which points to the sitemap.xml
• adding breadcrumb structured data using RDFa
• adding dataset structured data using JSON-LD, both on the landing page and collection pages.
• adding canonical meta tags to differentiate between languages (nl/en)
• adding Open Graph tags for social sharing (FB, Slack, X.com, Whatsapp, etc)

Type of change

• New feature

Checklist:

• I've double-checked the code in this PR myself
• I've left the code better than before (boy scout rule)
• The code is readable, comments are added that explain hard or non-obvious parts.
• I've expanded/improved the (unit) tests, when applicable
• I've run (unit) tests that prove my solution works
• There's no sensitive information like credentials in my PR