Make nimble documentation easier to find

[SpotlightKid]

Add a link to https://nim-lang.github.io/nimble/index.html to:

1. • https://nim-lang.org/documentation.html (section "Search" or "Other")
2. • https://nim-lang.org/docs/tools.html
3. • In the Github repo description of https://github.com/nim-lang/nimble
4. • In the readme of https://github.com/nim-lang/packages/
5. • In the header or footer of https://nimble.directory/

Example of how it is done on PyPI.org:

[xTrayambak]

AFAIK, https://nimble.directory isn't controlled by the Nim team.