docs: add new mix milestone

[jm-clius]

(Messed up commit history here, so will squash before merging).

While investigating the scope of: #340 we found three reasons why a second milestone for mix may be needed. This will also allow us to close the basic lightpush milestone soon.

• using discv5 discovery for mix is limited, as we reach the ENR size limit pretty soon (e.g. can't be used on fleet nodes)
• using rendezvous discovery for mix is possible, but require modifying the nim-libp2p implementation and adding rendezvous support for js-waku
• implementing js-mix in js-libp2p and subsequent integration in js-waku may introduce complexity risks too

For this reason I propose a second milestone to cover the discovery and js-waku work to improve mix use. At the same time we can add static configuration to allow the mix-for-lightpush milestone to be closed soon.

The addition of a static configuration task to the existing milestone also delayed its completion to beyond the initial expected 30 Sept due date, updated to 10 Oct.

WDYT @chaitanyaprem @fryorcraken?

[chaitanyaprem]

LGTM, Thanks!

[fryorcraken]

couple of comments, LGTM otherwise

[fryorcraken]

#291 (comment)

Please update commitment towards including mix in Waku API for js and nim