Update docs extraction script to handle preset titles

[rlenferink]

This updates the docs extraction script to be able to handle preset properties (e.g. titles); meaning after a release this script can be called with the released version and documentation will be pre-processed automatically.

Tested using:

[root@centos8 test]# ./extract_celix_docs.sh 1.0.0
'source/./test/bla.md' -> 'target/1.0.0/celix/./test/bla.md'
'source/./bla2.md' -> 'target/1.0.0/celix/./bla2.md'
'source/./bla.md' -> 'target/1.0.0/celix/./bla.md'

[root@centos8 test]# cat source/bla2.md
# Title
Description
[root@centos8 test]# cat target/1.0.0/celix/bla2.md
---
title: bla2.md
type: celix-doc
version: 1.0.0
---

# Title
Description

[root@centos8 test]# cat source/test/bla.md
---
title: Bla die Bla / Test
---

# Title
Description
[root@centos8 test]# cat target/1.0.0/celix/test/bla.md
---
title: Bla die Bla / Test
type: celix-doc
version: 1.0.0
---

# Title
Description

[root@centos8 test]# cat source/bla.md
---
title: Bla die ba 2
type: celix-alternative-doc
---

# Title
Description
[root@centos8 test]# cat target/1.0.0/celix/bla.md
---
title: Bla die ba 2
type: celix-alternative-doc
version: 1.0.0
---

# Title
Description

Xref: apache/celix#271

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