Extension for asciidoctor files

We are currently using a mixture of mostly `.txt` and `.asciidoc` (and a couple of `.adoc` files). Vulkan has recently started `.adoc` everywhere (including for generated files) to replace mostly `.txt` files. Should we follow suit?

Review the repository's existing .txt, .asciidoc, and .adoc sources, including generated files, and compare them with the Vulkan convention described in the issue. Done means deciding whether to standardize on .adoc and documenting the resulting scope and migration plan.

Written by the indexing model from the issue text.