doc: Introduce Inheritance Addendum to TSM Spec (#7876)

This introduces an addendum to the Terminal Settings Model spec that
covers inheritance and fallback. Basically, settings objects will now
have a reference to a parent object. If the settings object does not
have a setting defined, it will ask its parent to resolve the value. A
parent is set using the `Clone()` function. `Copy()` is used to copy the
value and structure of the settings model, whereas `Clone()` is used to
copy a reference to the settings model and build an inheritance tree.

## References
#6904 - Terminal Settings Model Spec
#1564 - Settings UI

.github/actions/spell-check/dictionary/dictionary.txt

@@ -157078,6 +157078,7 @@ geek
 geekier
 geekiest
 geeks
+geeksforgeeks
 geeky
 geelbec
 geelbeck

.github/actions/spell-check/excludes.txt

@@ -45,6 +45,7 @@ SUMS$
 \.tar$
 \.tgz$
 \.ttf$
+\.vsdx$
 \.woff
 \.xcf$
 \.xls