Improve documentation of operators

[moble]

I've always found the documentation of operators a little disjointed and incomplete; this PR aims to improve the situation.

• Add missing items to the precedence table
  • Adjoint operator goes after :: and before exponentiation
  • Juxtaposition of numeric literals goes after unary operators and before bitshifts
  • Arrows go between || and ?
  • New footnote explains exceptions to exponentiation precedence
  • New footnote explains non-associativity of comparisons, but notes chaining
• Add more complete lists of operators
  • Include all operators defined in Base
  • Include operators that are available but not defined in Base, set off by parentheses
• Operator suffixes
  • Note that new operators can be defined with suffixes
  • Explain what those prefixes are
  • Show an example
• Provide some links between disjoint pages
  • Link from Manual > Variables to operator section
  • Link from Manual > Integers and Floating-Point Numbers to operator section
  • Link from Manual > Functions to operator section
  • Link back to functions page when noting that operators are functions.

[moble]

Given that the documentation previously said that a space was required in expressions like the +ᵃx discussed above, I guess we would argue that just parsing it as +ᵃ x is actually a bug, which can be fixed by modifying the parser. As such, I don't want to weaken that language, so I've reverted my change to keep the stronger language.