Add assets.hostname property in header and nav partials

[samueleresca]

Related to apache/pekko-site#95 suppress #126

There are link in the paradox theme where we don't prepend the assets.hostname property. This cause some 404 on the assets of https://pekko.apache.org. For example:

• https://pekko.apache.org/docs/pekko/current/
• https://pekko.apache.org/docs/pekko-http/current/

Test locally

To test this locally:

1. Build and run sbt publishLocal
2. Specify the snapshot version in a downstream projects (e.g.: incubator-pekko-http, incubator-pekko-grpc)
3. Check the target/paradox/site/main/ folder

[pjfanning]

lgtm

If we get a new snapshot, I can try it out locally.

[samueleresca]

lgtm

If we get a new snapshot, I can try it out locally.

Proceeding by merging and we can try this in the downstream modules. I verified the changes locally by publishing it locally. Note that the resulting html should be published in the target/paradox/site/main/ folder