Documentation: Update Custom Pens documentation links

[tobybaratta]

Issue:

• The README contains a broken link to a not-on-main file documenting how to add custom pens. (That document was archived.)
• The README hard-links to the master branch of the EA folder rather than to whatever branch the GH user is on. I have made that link relative, so that when looking at a different branch, the links are relative to the branch the user is searching around on.

Fix:

• Pens are now handled in settings with a different YouTube video showing how to set them up (I think).
• Adds links to the Hardware Eraser functionality & Auto Draw for Pen.

Open Question/Discussion:

• I think it might be useful to link to the freestyle docs here still, or have some sort of master "Alternative Pens" link that still shows how to do the JSON setup as opposed to the UI setup? Not sure if necessary?
• In terms of pen setup docs, I just see issue Quality of Life Improvement - Optimizing UX for writing with pen #986 (with its screenshot) and of course the video setup. I'm still a beginner on Obsidian stuff but am open to help with writing up some more of the docs that are in YT videos, if that would be helpful to you.

Thanks for all your work! Please let me know if I can answer any questions!

[zsviczian]

Thanks. Any help with documentation is highly appreciated. Things have changed over time, and I do struggle to keep documentation up to date.