Update low/high level api docs

[davidyuk]

This PR is supported by the Æternity Foundation

[dincho]

Not sure why you're using absolute links to external services/docs while you can use relative to the repo. You plan to update the docs on each release version?

Also the external files examples doesn't make much sense as it's short and can be inlined for better readability flow, otherwise one have to click around.
The same question/comment applies here for absolute urls.

[davidyuk]

can be inlined for better readability flow

they are actually inlined by GitHub

and I also make it work in MkDocs. The problem is that there are only absolute links supported as I understand.

Also, it requires to specify lines, so I'm referencing the commit because if I add a line, it won't show in the docs.
Keeping example code in separate files also allow testing them, to ensure that they are still functional.

[davidyuk]

Not sure why you're using absolute links to external services/docs while you can use relative to the repo.

For links like https://docs.aeternity.com/aepp-sdk-js/v14.0.0/api/functions/buildTx.html. I can't reference them relative to the repo because api docs are generated on the docs build step. Instead of v14.0.0 I can reference to latest, but I think this way I may break v14 docs by dropping buildTx in v15. So, I prefer to update the version in links manually than potentially have broken links.