[pitivi] docs: Add instructions for inspecting the UI

[Alexandru Băluț <alexbalut src gnome org>]

commit b48b0ae23db3ddcdfca1cae8a5af17782f37151e
Author: Alexandru Băluț <alexandru balut gmail com>
Date:   Thu Jan 21 20:47:12 2021 +0100

    docs: Add instructions for inspecting the UI

 docs/Inspecting_UI.md | 16 ++++++++++++++++
 docs/sitemap.txt      |  1 +
 2 files changed, 17 insertions(+)
---
diff --git a/docs/Inspecting_UI.md b/docs/Inspecting_UI.md
new file mode 100644
index 000000000..d8c6773b1
--- /dev/null
+++ b/docs/Inspecting_UI.md
@@ -0,0 +1,16 @@
+---
+short-description: How to enable and use the GTK Inspector
+...
+
+# Hacking the UI
+
+The [GTK
+Inspector](https://developer.gnome.org/gtk4/stable/interactive-debugging.html)
+allows changing the widgets properties on the fly.
+
+The GTK Inspector can be activated by pressing `Ctrl-Shift-i` only if
+enabled in settings with:
+
+```
+(ptv-flatpak) $ ptvenv gsettings set org.gtk.Settings.Debug enable-inspector-keybinding true
+```
diff --git a/docs/sitemap.txt b/docs/sitemap.txt
index 650d6994d..1ad591e6c 100644
--- a/docs/sitemap.txt
+++ b/docs/sitemap.txt
@@ -8,6 +8,7 @@ index.md
                Git.md
                Coding_style_guide.md
                Command_line_tools.md
+               Inspecting_UI.md
        Testing.md
                QA_Scenarios.md
        Architecture.md