Title screen: Replace logo in main menu (#1341)

Set expand mode to "keep size". And change the container sizing
horizontally to "center of reduction" (like the vertical one).

Remove `.license` file, so it should now match rule path
"assets/first_party/**/*.png" in the REUSE.toml file, defining the logo
as CC-BY-SA-4.0 by The Threadbare Authors.

Update the licensing documentation that used this logo as example.

Helps #1338

Author: manuq

assets/first_party/logo/threadbare-logo.png

@@ -163,11 +163,9 @@ wholly by AI, provided that:
    generation or provided another asset as input to the AI tool, and ideally the
    model (if known) and prompt used.
 
-For example, the current
-[main menu logo](../assets/first_party/logo/threadbare-logo.png)
-was generated with Midjourney, with no modifications. It is accompanied by a
-[`.license` file](../assets/first_party/logo/threadbare-logo.png.license)
-which reads:
+For example, the previous main menu logo at `assets/first_party/logo/threadbare-logo.png`
+was generated with Midjourney, with no modifications. It was accompanied by a
+`threadbare-logo.png.license` file in the same folder which said:
 
 ```
 SPDX-FileCopyrightText: The Threadbare Authors

assets/first_party/logo/threadbare-logo.png.license

@@ -30,9 +30,9 @@ size_flags_horizontal = 3
 [node name="Logo" type="TextureRect" parent="LogoButtonSplit/LogoContainer"]
 texture_filter = 2
 layout_mode = 2
+size_flags_horizontal = 4
 size_flags_vertical = 4
 texture = ExtResource("1_hgbs1")
-expand_mode = 5
 stretch_mode = 4
 
 [node name="ButtonBoxMargins" type="MarginContainer" parent="LogoButtonSplit"]