Improve embed function for docs

[RealAnna]

this function here embeds yaml files from a branch but requires the files to already exist in the branch to work.

This is needed for file outside of the docs since they won't be copyied in the docker container that generates the doc website. However for those yaml file in the docs folder, eg here we should be able to use relative paths so that one can add the file in the same pr it is embedded.

AC

• I can use {{< embed path="/docs/mynew.yaml" >}} without the script prepending "https://raw.githubusercontent.com/keptn/lifecycle-toolkit/"
• for all path outside docs the script keeps on using "https://raw.githubusercontent.com/keptn/lifecycle-toolkit/"
  ex. {{< embed path="/lifecycle-operator/config/samples/myhastobetherealready.yaml" >}} will be embedded as https://raw.githubusercontent.com/keptn/lifecycle-toolkit/lifecycle-operator/config/samples/myhastobetherealready.yaml

[thisthat]

with #2398, a new shortcode has been introduced docsembed

[mowies]

The team agreed to close this since 2 separate shortcodes are good enough for now.