add matrix.org homeserver experimental features policy

I have raised the question about such a policy with the governing board. While there isn't a decision, the Foundation side was able to share some information about the current "rule of thumb" policy in place, including some considerations taken into account.

Upon [further discussion](https://matrix.to/#/%21sWpnrYUMmaBrlqfRdn%3Amatrix.org/%245GpiaV-A3eLiAoKLIvT3vfIOtTzwy1ml0zZVw1OEPoQ?via=matrix.org&via=usacali.chat&via=matrix.at.grovecastle.me) in the "Office of the Matrix.org Foundation" room, RichvdH shared this page from the Element backend team's (who are contracted to operate the matrix.org homeserver) internal handbook: https://gist.github.com/richvdh/d7e83516207efbe9b31c4e2529a4132d

The discussion has led to the provisional agreement that we add the suitable parts to an appropriate place under matrix.org/homeserver, after which the backend team will reference it from their internal docs and also maintain it (we should hence add them as codeowners of that section as we add it).

The backend team presumabily is `@matrix-org/synapse-core`.

Read the linked Element backend handbook and inspect the existing content under matrix.org/homeserver to identify the appropriate location for the policy. Add the suitable policy information and update the relevant code-ownership entry for @matrix-org/synapse-core; done means the policy is published and the backend team can maintain that section.

Written by the indexing model from the issue text.