Documentation - Repaired missing/broken links and fixed mkdocs errors

[plumpdolphin]

Overview

This is a pull aimed at fixing all remaining broken hyperlinks, images, and missing pages in the documentation. This is aimed at fixing the issue found here: #76. It's not a lot of lines, but there's a good bit to go through!

Changes

1. I have done so by updating the linking style to other pages to relative links to the markdown instead of absolute links to the github.io pages. This allows MKDocs to recognize and view the pages locally, and also allows more flexibility because the documentation can be uploaded anywhere, under any nested path. So, further changes to the naming won't require additional work updating the link addresses.

2. I also fixed one broken image in the process and updated it's link accordingly (found under Send To Unreal > Supported Extensions > Create Post Import Groom Assets).

3. I added missing pages to the navigation, including the index pages for the project, send2ue, and ue2rigify. Previously, once left, these pages could not be found again graphically.

4. Additionally, I added 3 pages which were not added to the navigation related to User Interface. They appear to be complete and may be useful to some users, but if these are intentionally left out, I would be happy to revise this. They could also be placed under not_in_nav: in the configuration to keep them hidden but silence the warning.

5. Lastly, in-order to unify the link structure, I set the site_name for the send2ue and ue2rigify to their corresponding folder names respectively, instead of Send to Unreal and UE to Rigify. Reason being: currently the index linked to for Send to Unreal is https://poly-hammer.github.io/BlenderTools/send2ue, however all endpoints nested under /BlenderTools/send2ue link instead to /BlenderTools/send-to-unreal/<example>/. These changes I have done will restore continuity, but if the path send-to-unreal is preferred, this can be fixed by renaming the folder rather than changing the site_name value.

Conclusion

Whew, that was a mouthful. This is my best take at updating the documentation, and in my testing, it fixes all the broken resources and leaves the MKDocs output squeaky-clean! However, I do know that numbers 4 and 5 especially may require modification or removal, so I am happy to make any of those changes upon request!

[jack-yao91]

Looks good to me. Thanks for fixing these issues!

[JoshQuake]

Beautiful, thank you for your contribution!