[1/4] Added: Documentation for detecting mod updates. (#2072)

* WIP: Documentation for Update Detection

* Added: Additional Edge Cases to Update Logic Docs

* Added: Extra case of `Archived in the Middle`

* Fixed: Indentation for `file_updates` field.

* Finalized 'Updating Mods' doc with simplified Implementation requested.

* Fixed: Minor Notes from older Research Doc

* Fixed: Added Missing 'Updating Mods' mkdocs sidebar item.

* Added: Missing 'newer' word

docs/developers/decisions/backend/0019-updating-mods.md

@@ -0,0 +1,40 @@
+# Updating Mods
+
+!!! info "This is a design document detailing the steps taken to update mods."
+
+A corresponding research document (original design) can be found on a [separate page][research-doc].
+
+## General Approach
+
+!!! tip "First read the [Problem Statement] in the [Research Document]"
+
+The requested approach (from business) has been to maximize the use of the V2 API,
+as opposed to programming against the legacy V1 API.
+
+To achieve this, we will [NOT use the `file_updates` array from V1 API's Querying Mod Files][querying-mod-files];
+instead choosing to opt to wait until backend decides their future plans with
+respect to 'Mods 2.0' project, and how mod updates will be handled in V2 API in the future.
+
+For now, we will:
+
+- [1. Determine Updated Mod Pages], to update our local cache.
+- [2. Multi Query Pages], for update mod pages with a 'cache miss'.
+
+## Displaying Mod Updates
+
+!!! info "We display all files on a given mod page that are more recent (file upload time) than the user's file."
+
+Although uncommon this may include:
+
+- Files for other mods on same mod page.
+- Older files (if uploaded out of order).
+
+We will for now rely on *users' common sense* to identify whether a file is an 
+update to a previous file or not. Until site decides on future plans.
+
+[Problem Statement]: ../../misc/research/00-update-implementation-research.md#problem-statement
+[1. Determine Updated Mod Pages]: ../../misc/research/00-update-implementation-research.md#1-determine-updated-mod-pages
+[2. Multi Query Pages]: ../../misc/research/00-update-implementation-research.md#multi-query-pages
+[querying-mod-files]: ../../misc/research/00-update-implementation-research.md#2-querying-mod-files
+[Research Document]: ../../misc/research/00-update-implementation-research.md
+[research-doc]: ../../misc/research/00-update-implementation-research.md