RFR: 8337287: Update image in javax.swing.text.Document.insert

[Alexey Ivanov]

This changeset updates the image in the documentation for the `Document.insert` method. The image in `Document.remove` was updated by [JDK-4622866](https://bugs.openjdk.org/browse/JDK-4622866) in PR #15701.

Now the illustration of inserting looks similar to removing. The new image is in SVG format.

For reference:

- [`Document.insert` in JDK 22](https://docs.oracle.com/en/java/javase/22/docs/api/java.desktop/javax/swing/text/Document.html#insertString(int,java.lang.String,javax.swing.text.AttributeSet));
- [Updated docs for JDK 24](https://cr.openjdk.org/~aivanov/8337287/api/java.desktop/javax/swing/text/Document.html#insertString(int,java.lang.String,javax.swing.text.AttributeSet)).

As in the case with `remove`, I marked up to classes and members with {@code}.

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Commit messages:
 - Remove Document-insert.gif
 - 8337287: Update image in javax.swing.text.Document.insert

Changes: https://git.openjdk.org/jdk/pull/20376/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20376&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337287
  Stats: 164 lines in 3 files changed: 150 ins; 0 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/20376.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20376/head:pull/20376

PR: https://git.openjdk.org/jdk/pull/20376