[BUG] mkdocs only sees generated files after the second generation

[bczsalba]

Describe the bug
When generating from a clear tree (e.g. docs/assets is empty), the first run will have mkdocs complain about links to non-detected files:

To Reproduce
Steps to reproduce the behavior:

1. Run rm -rf docs/assets/*
2. Run mkdocs serve
3. Look at the terminal output and see warnings

If the development server is killed and restarted, the problem goes away.

Expected behavior
The files should be detected on first run.

Platform info
Browser: All

PTG:

PyTermGUI version 6.2.1

System details:
    Python version: 3.9.12
    $TERM:          xterm-kitty
    $COLORTERM:     truecolor
    Color support:  ColorSystem.TRUE
    OS Platform:    macOS-12.3.1-arm64-arm-64bit

[bczsalba]

This could be fixed by adding File instances to the Files collection MkDocs generates, but documentation around it is sparse and I haven't figured out how to yet.