Populate MkPages from SUMMARY.md

MkNavs can parse index files in SUMMARY.md style (as popularized by mkdocs-literate-nav).

Given the following folder structure:

Directory tree

📂 `nav_tree`
┣━━ 📂 `test_folder`
┃   ┣━━ 📝 `markdown-exec.md (508 Bytes)`
┃   ┗━━ 📝 `sub_1.md (287 Bytes)`
┣━━ 📂 `test_subnav`
┃   ┣━━ 📝 `subnav_page_1.md (132 Bytes)`
┃   ┣━━ 📝 `subnav_page_2.md (1.7 kB)`
┃   ┗━━ 📝 `SUMMARY.md (72 Bytes)`
┣━━ 📝 `SUMMARY.md (162 Bytes)`
┗━━ 📝 `test_file.md (55 Bytes)`

and a root SUMMARY.md looking like:

Root SUMMARY.md

* Test nested
    * [General] (test_folder/sub_1.md)
    * [Markdown-Exec] (test_folder/markdown-exec.md)
* [Test Subnav] (test_subnav/)
* [Test file] (test_file.md)