Replace tempo templates with tempel

[bbatsov]

Tempo mode (which adoc-mode uses internally for adoc templates) is quite old and has pretty much no documentation. It seems that Tempel is a nice modern alternative https://github.com/minad/tempel

I guess using Yasnippet or similar is another alternative we can consider.

[TobiasZawada]

The tempel-page has its own overview of alternatives: https://github.com/minad/tempel#alternatives

[bbatsov]

Tempel seems to be the past of least resistance, but I'm open to other suggestions as well.

[dunmaksim]

Project Tempel have a package tempel-collection like a yasnippet-snippets. I think, we can:

1. Delete all templates from adoc-mode.el and create PR to tempel-collection package.
2. Add to README.md chapter about using adoc-mode with tempel and tempel-collection.

[bbatsov]

@dunmaksim I like this suggestion.

[tripplilley]

Project Tempel have a package tempel-collection

@dunmaksim : Link is broken. Correct link is: https://github.com/Crandel/tempel-collection