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/webvtt/webvtt_example_01.vtt

@@ -0,0 +1,42 @@
+WEBVTT
+
+NOTE Copyright © 2019 World Wide Web Consortium. https://www.w3.org/TR/webvtt1/
+
+00:11.000 --> 00:13.000
+<v Roger Bingham>We are in New York City
+
+00:13.000 --> 00:16.000
+<v Roger Bingham>We’re actually at the Lucern Hotel, just down the street
+
+00:16.000 --> 00:18.000
+<v Roger Bingham>from the American Museum of Natural History
+
+00:18.000 --> 00:20.000
+<v Roger Bingham>And with me is Neil deGrasse Tyson
+
+00:20.000 --> 00:22.000
+<v Roger Bingham>Astrophysicist, Director of the Hayden Planetarium
+
+00:22.000 --> 00:24.000
+<v Roger Bingham>at the AMNH.
+
+00:24.000 --> 00:26.000
+<v Roger Bingham>Thank you for walking down here.
+
+00:27.000 --> 00:30.000
+<v Roger Bingham>And I want to do a follow-up on the last conversation we did.
+
+00:30.000 --> 00:31.500 align:right size:50%
+<v Roger Bingham>When we e-mailed—
+
+00:30.500 --> 00:32.500 align:left size:50%
+<v Neil deGrasse Tyson>Didn’t we talk about enough in that conversation?
+
+00:32.000 --> 00:35.500 align:right size:50%
+<v Roger Bingham>No! No no no no; 'cos 'cos obviously 'cos
+
+00:32.500 --> 00:33.500 align:left size:50%
+<v Neil deGrasse Tyson><i>Laughs</i>
+
+00:35.500 --> 00:38.000
+<v Roger Bingham>You know I’m so excited my glasses are falling off here.

tests/data/webvtt/webvtt_example_02.vtt

@@ -0,0 +1,15 @@
+WEBVTT
+
+NOTE Copyright © 2019 World Wide Web Consortium. https://www.w3.org/TR/webvtt1/
+
+00:00.000 --> 00:02.000
+<v.first.loud Esme>It’s a blue apple tree!
+
+00:02.000 --> 00:04.000
+<v Mary>No way!
+
+00:04.000 --> 00:06.000
+<v Esme>Hee!</v> <i>laughter</i>
+
+00:06.000 --> 00:08.000
+<v.loud Mary>That’s awesome!

tests/data/webvtt/webvtt_example_03.vtt

@@ -0,0 +1,57 @@
+WEBVTT
+
+62357a1d-d250-41d5-a1cf-6cc0eeceffcc/15-0
+00:00:04.963 --> 00:00:08.571
+<v Speaker A>OK,
+I think now we should be recording</v>
+
+62357a1d-d250-41d5-a1cf-6cc0eeceffcc/15-1
+00:00:08.571 --> 00:00:09.403
+<v Speaker A>properly.</v>
+
+62357a1d-d250-41d5-a1cf-6cc0eeceffcc/16-0
+00:00:10.683 --> 00:00:11.563
+Good.
+
+62357a1d-d250-41d5-a1cf-6cc0eeceffcc/17-0
+00:00:13.363 --> 00:00:13.803
+<v Speaker A>Yeah.</v>
+
+62357a1d-d250-41d5-a1cf-6cc0eeceffcc/78-0
+00:00:49.603 --> 00:00:53.363
+<v Speaker B>I was also thinking.</v>
+
+62357a1d-d250-41d5-a1cf-6cc0eeceffcc/113-0
+00:00:54.963 --> 00:01:02.072
+<v Speaker B>Would be maybe good to create items,</v>
+
+62357a1d-d250-41d5-a1cf-6cc0eeceffcc/113-1
+00:01:02.072 --> 00:01:06.811
+<v Speaker B>some metadata,
+some options that can be specific.</v>
+
+62357a1d-d250-41d5-a1cf-6cc0eeceffcc/150-0
+00:01:10.243 --> 00:01:13.014
+<v Speaker A>Yeah,
+I mean I think you went even more than</v>
+
+62357a1d-d250-41d5-a1cf-6cc0eeceffcc/119-0
+00:01:10.563 --> 00:01:12.643
+<v Speaker B>But we preserved the atoms.</v>
+
+62357a1d-d250-41d5-a1cf-6cc0eeceffcc/150-1
+00:01:13.014 --> 00:01:15.907
+<v Speaker A>than me.
+I just opened the format.</v>
+
+62357a1d-d250-41d5-a1cf-6cc0eeceffcc/197-1
+00:01:50.222 --> 00:01:51.643
+<v Speaker A>give it a try, yeah.</v>
+
+62357a1d-d250-41d5-a1cf-6cc0eeceffcc/200-0
+00:01:52.043 --> 00:01:55.043
+<v Speaker B>Okay, talk to you later.</v>
+
+62357a1d-d250-41d5-a1cf-6cc0eeceffcc/202-0
+00:01:54.603 --> 00:01:55.283
+<v Speaker A>See you.</v>