[FEATURE] allow embedding in an iframe

[micw]

[FEATURE] allow embedding in an iframe (including cross-domain) by using postmessage

Description

This patch adds "postmessage" to provide the hash to the outer IFrame, so that navigation + deep linking works cross-domain

Motivation and Context

We embedd the map in our homepage. The map uses a different domain: https://freifunk-leipzig.de/map/

How Has This Been Tested?

Firefox. Also tested by our users in the last few years ;-)

Screenshots/links:

Checklist:

• My code follows the code style of this project. (CI will test it anyway and also needs approval)
• My change requires a change to the documentation.
  • I have updated the documentation accordingly (added an example page and embedded/README.md)

[github-actions]

Preview deployed: https://freifunk.github.io/meshviewer/pr-preview/pr-378/

Updated for commit a7f8551.

[micw]

Demo: https://freifunk.github.io/meshviewer/pr-preview/pr-378/embed/

Testing:

1. select a node (or something else)
2. expected: the hash is propagated to the iframe
3. open an iframe with a hash
4. expected: the hash is propagated to the map