payable function modifier documentation not found

## Abstract

I cannot find documentation for `payable` in Solidity docs.

Not sure if this is because of lack of table of contents, per page table of contents in the documentation or this documentation simply does not exist.

Here is a Sphinx search https://docs.soliditylang.org/en/v0.7.4/search.html?q=payable&check_keywords=yes&area=default#

Here is a Google search https://www.google.com/search?hl=en&q=solidity%20payable

## Motivation

Not sure how Sphinx search works, but I think all Solidity keywords should be easily searchable.

I suggest restructuring the function modifier page so that it is easier to navigate and read.

Start by locating the Solidity documentation page covering function modifiers and checking how the Sphinx table of contents and search index expose it. Compare the existing treatment of `payable` with the other Solidity keywords, then define the documentation and navigation changes needed so the term is easy to find and understand.

Written by the indexing model from the issue text.