docs: add spdx tags to all .md files

I have no idea if this is going to cause rendering problems, and it is fairly
hard to check. So let's just merge this, and if it github markdown processor
doesn't like it, revert.

docs/ARCHITECTURE.md

@@ -2,6 +2,7 @@
 title: systemd Repository Architecture
 category: Contributing
 layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
 ---
 
 # Code Map

docs/AUTOMATIC_BOOT_ASSESSMENT.md

@@ -2,6 +2,7 @@
 title: Automatic Boot Assessment
 category: Booting
 layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
 ---
 
 # Automatic Boot Assessment

docs/BLOCK_DEVICE_LOCKING.md

@@ -2,6 +2,7 @@
 title: Locking Block Device Access
 category: Interfaces
 layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
 ---
 
 # Locking Block Device Access

docs/BOOT_LOADER_INTERFACE.md

@@ -2,6 +2,7 @@
 title: Boot Loader Interface
 category: Booting
 layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
 ---
 
 # The Boot Loader Interface

docs/BOOT_LOADER_SPECIFICATION.md

@@ -2,6 +2,7 @@
 title: Boot Loader Specification
 category: Booting
 layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
 ---
 
 # The Boot Loader Specification

docs/CGROUP_DELEGATION.md

@@ -2,6 +2,7 @@
 title: Control Group APIs and Delegation
 category: Interfaces
 layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
 ---
 
 # Control Group APIs and Delegation

docs/CODE_OF_CONDUCT.md

@@ -2,6 +2,7 @@
 title: systemd Community Conduct Guidelines
 category: Contributing
 layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
 ---
 
 # The systemd Community Conduct Guidelines

docs/CODE_QUALITY.md

@@ -2,6 +2,7 @@
 title: Code Quality Tools
 category: Contributing
 layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
 ---
 
 # Code Quality Tools

docs/CODING_STYLE.md

@@ -2,6 +2,7 @@
 title: Coding Style
 category: Contributing
 layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
 ---
 
 # Coding Style

docs/CONTAINER_INTERFACE.md

@@ -2,6 +2,7 @@
 title: Container Interface
 category: Interfaces
 layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
 ---
 
 # The Container Interface

docs/CONTRIBUTING.md

@@ -2,6 +2,7 @@
 title: Contributing
 category: Contributing
 layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
 ---
 
 # Contributing

docs/CONVERTING_TO_HOMED.md

@@ -2,6 +2,7 @@
 title: Converting Existing Users to systemd-homed
 category: Users, Groups and Home Directories
 layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
 ---
 
 # Converting Existing Users to systemd-homed managed Users

docs/COREDUMP_PACKAGE_METADATA.md

@@ -2,6 +2,7 @@
 title: Package Metadata for Core Files
 category: Interfaces
 layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
 ---
 
 # Package Metadata for Core Files

docs/DESKTOP_ENVIRONMENTS.md

@@ -2,6 +2,7 @@
 title: Desktop Environment Integration
 category: Concepts
 layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
 ---
 
 # Desktop Environments

docs/DISCOVERABLE_PARTITIONS.md

@@ -2,6 +2,7 @@
 title: Discoverable Partitions Specification
 category: Concepts
 layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
 ---
 # The Discoverable Partitions Specification
 

docs/DISTRO_PORTING.md

@@ -2,6 +2,7 @@
 title: Porting systemd To New Distributions
 category: Concepts
 layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
 ---
 
 # Porting systemd To New Distributions

docs/ENVIRONMENT.md

@@ -2,6 +2,7 @@
 title: Known Environment Variables
 category: Interfaces
 layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
 ---
 
 # Known Environment Variables

docs/GROUP_RECORD.md

@@ -2,6 +2,7 @@
 title: JSON Group Records
 category: Users, Groups and Home Directories
 layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
 ---
 
 # JSON Group Records

docs/GVARIANT-SERIALIZATION.md

@@ -2,6 +2,7 @@
 title: GVariant D-Bus Message Serialization
 category: Interfaces
 layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
 ---
 
 # GVariant D-Bus Message Serialization

docs/HACKING.md

@@ -2,6 +2,7 @@
 title: Hacking on systemd
 category: Contributing
 layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
 ---
 
 # Hacking on systemd