add page title to Content Block Manager home page

This sets the beginning of the page title - the
second half, the ` - GOV.UK Content Block Manager`
is set by the shared layout publishing component
[1] so it can't be changed.

The naming discussed on Slack
https://gds.slack.com/archives/C062Z1FA2ES/p1736500050626609?thread_ts=1736442857.222039&cid=C062Z1FA2ES
as making sense for now - it's what we refer to
the page as in guidance - but we may change later
if more suitable name becomes apparent.

[1]https://github.com/alphagov/govuk_publishing_components/blob/main/app/views/govuk_publishing_components/components/_layout_for_admin.html.erb

lib/engines/content_block_manager/app/views/content_block_manager/content_block/documents/index.html.erb

@@ -1,6 +1,8 @@
 
 <% content_for :title_margin_bottom, 6 %>
 
+<% content_for :page_title, "Home" %>
+
 <% if @error_summary_errors %>
   <%= render "govuk_publishing_components/components/error_summary", {
     title: "There is a problem",

lib/engines/content_block_manager/features/content_block_manager.feature

@@ -3,6 +3,7 @@ Feature: Content block manager
   Scenario: Correct layout is used
     Given I am a GDS admin
     When I visit the Content Block Manager home page
+    Then I should see the object store's home page title
     Then I should see the object store's title in the header
     And I should see the object store's navigation
     And I should see the object store's phase banner

lib/engines/content_block_manager/features/step_definitions/content_block_manager_steps.rb

@@ -788,6 +788,10 @@ def click_save_and_continue
   expect(page).to have_selector(".govuk-header__product-name", text: "Content Block Manager")
 end
 
+Then(/^I should see the object store's home page title$/) do
+  expect(page).to have_title "Home - GOV.UK Content Block Manager"
+end
+
 And(/^I should see the object store's navigation$/) do
   expect(page).to have_selector("a.govuk-header__link[href='#{content_block_manager.content_block_manager_root_path}']", text: "Dashboard")
 end