External contribution guidelines

## Page
https://docs.soliditylang.org/en/latest/contributing.html

## Abstract
The contribution guidelines does not describe the appropriate steps for external contribution. Apparently, an external contribution will be summarily rejected if the issue is not triaged or selected for development, such as [here](https://github.com/ethereum/solidity/pull/15764) and [here](https://github.com/ethereum/solidity/pull/14864).

## Pull request
I would've created a PR to fix the page, but
1) I had no assurance that it wouldn't simply be closed because it was not triaged or selected for development.
2) I'm not sure if there are other requirements, such as the issue being assigned that need to be added.

Read the contributing page and the two linked pull requests first to compare the current guidance with how external contributions were handled. Confirm the project’s triage, selection, assignment, and other stated requirements with maintainers, then update the page so the external contribution steps and acceptance conditions are explicit.

Written by the indexing model from the issue text.