Docs: add dates to manual and stdlib
Docs: cleanup dates chapter. Closes #8032

- Remove reference to Dates tests
- Use Sphinx :mod: reference to Dates module
Allow all geometric shapes as valid characters
Defines joins (⨝ ⟕ ⟖ ⟗) and antijoin (▷) as valid infix operators

Note: ▷ is semantically a geometric shape, but I could not find a separate character for antijoin