feat: add a backend parser for WebVTT files (#2288)

* feat: add a backend parser for WebVTT files

Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>

* docs: update README with VTT support

Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>

* docs: add description to supported formats

Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>

* chore: upgrade docling-core to unescape WebVTT in markdown

Pin the new release of docling-core 2.48.2.
Do not escape HTML reserved characters when exporting WebVTT documents to markdown.

Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>

* test: add missing copyright notice

Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>

---------

Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>

tests/data/groundtruth/docling_v2/webvtt_example_01.vtt.md

@@ -0,0 +1,51 @@
+00:11.000 --> 00:13.000
+
+Roger Bingham:  We are in New York City
+
+00:13.000 --> 00:16.000
+
+Roger Bingham:  We’re actually at the Lucern Hotel, just down the street
+
+00:16.000 --> 00:18.000
+
+Roger Bingham:  from the American Museum of Natural History
+
+00:18.000 --> 00:20.000
+
+Roger Bingham:  And with me is Neil deGrasse Tyson
+
+00:20.000 --> 00:22.000
+
+Roger Bingham:  Astrophysicist, Director of the Hayden Planetarium
+
+00:22.000 --> 00:24.000
+
+Roger Bingham:  at the AMNH.
+
+00:24.000 --> 00:26.000
+
+Roger Bingham:  Thank you for walking down here.
+
+00:27.000 --> 00:30.000
+
+Roger Bingham:  And I want to do a follow-up on the last conversation we did.
+
+00:30.000 --> 00:31.500
+
+Roger Bingham:  When we e-mailed—
+
+00:30.500 --> 00:32.500
+
+Neil deGrasse Tyson:  Didn’t we talk about enough in that conversation?
+
+00:32.000 --> 00:35.500
+
+Roger Bingham:  No! No no no no; 'cos 'cos obviously 'cos
+
+00:32.500 --> 00:33.500
+
+Neil deGrasse Tyson:  *Laughs*
+
+00:35.500 --> 00:38.000
+
+Roger Bingham:  You know I’m so excited my glasses are falling off here.