Add sphinx ext, case check image ref and file names

[duianto]

An image appears broken/not found, when there's a case mismatch between the image reference and file name.

It's happened before: #7 (comment)

The issue isn't visible in Windows and MacOS, because they use case-insensitive file systems.
But the issue appears on the live docs page. It's probably running on a Linux server (case-sensitive).

This extension shows a sphinx build error when there's a case mismatch.

ERROR: Case mismatch: image ref and file name, at C:\Users\username\Documents\projects\charachorder\docs\docs\Device Manager.rst:403
    reference: ManagerSettingsAutocorrect.png
    file name: ManagerSettingsAutoCorrect.png
ERROR: Case mismatch: image ref and file name, at C:\Users\username\Documents\projects\charachorder\docs\docs\Master Forge.rst:325
    reference: DM-apply-update-button-M4G.png
    file name: DM-apply-update-button-m4g.png
ERROR: Case mismatch: image ref and file name, at C:\Users\username\Documents\projects\charachorder\docs\docs\Master Forge.rst:342
    reference: FW-connect-button.jpg
    file name: FW-connect-button.JPG
ERROR: Case mismatch: image ref and file name, at C:\Users\username\Documents\projects\charachorder\docs\docs\Master Forge.rst:385
    reference: FW-connect-button.jpg
    file name: FW-connect-button.JPG

[duianto]

This extension checks the images in assets/images.
There are two images outside that directory.
assets/cce/pinout.png
assets/serial/serialterminal.png

They are moved into assets/images in this PR:
Move two images into assets/images dir #129