Update theme gem and document PLATFORM_NAME variable

Why these changes are being introduced:

The GDT project added a feature to the theme gem to conditionally
include the platform name in the header if the `PLATFORM_NAME`
environment variable is supplied.

Relevant ticket(s):

https://mitlibraries.atlassian.net/browse/GDT-124

How this addresses that need:

This updates the theme gem to the v1.2 tag, which includes the new
feature, and documents the PLATFORM_NAME variable.

Side effects of this change:

None. The app will continue to use the standard header if
PLATFORM_NAME is not supplied.

Gemfile

@@ -21,7 +21,7 @@ gem 'importmap-rails'
 # Build JSON APIs with ease [https://github.com/rails/jbuilder]
 gem 'jbuilder'
 
-gem 'mitlibraries-theme', git: 'https://github.com/mitlibraries/mitlibraries-theme', tag: 'v1.1'
+gem 'mitlibraries-theme', git: 'https://github.com/mitlibraries/mitlibraries-theme', tag: 'v1.2'
 
 # Use the Puma web server [https://github.com/puma/puma]
 gem 'puma', '>= 5.0'

Gemfile.lock

@@ -1,7 +1,7 @@
 GIT
   remote: https://github.com/mitlibraries/mitlibraries-theme
-  revision: 93b931b802485f9e35a6878f957b3fd88ae3b294
-  tag: v1.1
+  revision: bcbe5d3de36a92d275085a045c5c4d8f30f33e62
+  tag: v1.2
   specs:
     mitlibraries-theme (1.0.2)
       rails (>= 6, < 8)

README.md

@@ -1,11 +1,17 @@
 # TACOS: Tool for Analyzing and Categorization Of Searchterms
 
-## Required Environment Variables
+## Environment Variables
+
+### Required
 
 `LINKRESOLVER_BASEURL`: base url for our link resolver. `https://mit.primo.exlibrisgroup.com/discovery/openurl?institution=01MIT_INST&rfr_id=info:sid/mit.tacos.api&vid=01MIT_INST:MIT` is probably the best value unless you are doing something interesting.
 
 `UNPAYWALL_EMAIL`: email address to include in API call as required in their [documentation](https://unpaywall.org/products/api). Your personal email is appropriate for development. Deployed and for tests, use the timdex moira list email.
 
+### Optional
+
+`PLATFORM_NAME`: The value set is added to the header after the MIT Libraries logo. The logic and CSS for this comes from our theme gem.
+
 ## Documentation
 
 [Architecture Decisions](docs/architecture-decisions/)