doc: remove recommendation to use node-eps

[richardlau]

https://github.com/nodejs/node-eps has been archived and new proposals
can no longer be opened there. Remove the corresponding recommendation
to use it.

Refs: nodejs/TSC#335

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

[nodejs-github-bot]

@richardlau build started: https://ci.nodejs.org/blue/organizations/jenkins/node-test-pull-request-lite-pipeline/detail/node-test-pull-request-lite-pipeline/1051/pipeline

[refack]

I know we archived nodejs/node-eps, but maybe we can find some words to put in its stead.
Maybe something like:

For new modules that involve significant effort, non-trivial additions to Node.js or significant new capabilities, a preceding "RFC" issue is recommended but not required. Such an issue would present the planned design and implementation of the new feature, and could allow for some preliminary discussion to happen before significant time is invested. 

Maybe /s/RFC/Tracking

[danbev]

Landed in 9b34dfe.