docs: cross-link concept pages and API pages in See also sections

Add See also links in both directions between the concept pages (features,
home, integration) and the API pages, write API links in code font, give every
entry a comment after a dash, and show the callback event figure on the parser
callbacks page.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/front.md

@@ -50,9 +50,10 @@ Constant.
 
 ## See also
 
-- [back](back.md) to access the last element
-- [basic_json_view::front](../basic_json_view/front.md) - the same access on a zero-copy view (document order, not
+- [`back`](back.md) - to access the last element
+- [`basic_json_view::front`](../basic_json_view/front.md) - the same access on a zero-copy view (document order, not
   sorted; duplicate keys are visited)
+- [Element Access](../../features/element_access/index.md) - overview of all ways to access elements
 
 ## Version history