Update `s2sConfig` docs with configurable URLs

[rmloveland]

As described in #477, this will now look something like:

pbjs.setConfig({
    cache: {
        url: "https://prebid.adnxs.com/pbc/v1/cache"
    },
    s2sConfig: {
        ...
        endpoint: "https://prebid.adnxs.com/pbs/v1/auction",
        syncEndpoint: "https://prebid.adnxs.com/pbs/v1/cookie_sync",
        ...
    }
});

[rmloveland]

All mentions:

ack --ignore-dir=_site 's2sConfig:'
dev-docs/get-started-with-prebid-server.md
73:        s2sConfig: {

dev-docs/modules/s2sTesting.md
43:  s2sConfig: {
116:  s2sConfig: {
141:  s2sConfig: {
181:  s2sConfig: {

dev-docs/publisher-api-reference.md
1165:    s2sConfig: {

[rmloveland]

@bretg should the examples of s2sConfig on this s2s testing module page also be updated to use the configurable URLs? E.g.,

endpoint: 'https://prebid.adnxs.com/pbs/v1/auction'
syncEndpoint: 'https://prebid.adnxs.com/pbs/v1/cookie_sync'

I assume the answer is yes, but I'm not familiar with the testing syntax and wanted to make sure since you submitted that doc.

[rmloveland]

This issue is addressed by e698b4e