Add SPDX markers for a bunch of remaining trivial files

Down to only two missing files for full REUSE compliance now.

src/knowledgedb-generator/README

@@ -22,3 +22,6 @@ Extract the zip file into this folder.
 (3) Run the code generator
 
 Run `make rebuild-knowledgedb` in the build dir of this folder.
+
+SPDX-FileCopyrightText: none
+SPDX-License-Identifier: CC0-1.0

src/knowledgedb-generator/timezones.qgs.license

+SPDX-License-Identifier: CC0-1.0
+SPDX-FileCopyrightText: none

src/lib/Mainpage.dox

+/*
+  SPDX-FileCopyrightText: none
+  SPDX-License-Identifier: CC0-1.0
+*/
+
 /**
  * @namespace KItinerary
  * Classes for reservation/travel data models, data extraction and data augmentation.

src/lib/extractors/README

 Development tip: symlink this folder to $PREFIX/share/kitinerary/extractors
 It will then take priority over the compiled-in files allowing you to test
 script changes without recompilation.
+
+SPDX-FileCopyrightText: none
+SPDX-License-Identifier: CC0-1.0

src/lib/vdv/certs/NOTES

 The CA certs in here are retrieved from ldap://ldap-vdv-ion.telesec.de:389/cn=<CAR>,ou=VDV%20KA,o=VDV%20Kernapplikations%20GmbH,c=de
 using the vdv-cert-downloader tool in this folder, with all their signatures or other for us irrelevant data stripped, leaving just the
 public keys.
+
+SPDX-FileCopyrightText: none
+SPDX-License-Identifier: CC0-1.0