Reorganize difflib documentation

[encukou]

Documentation

The structure of difflib docs can be improved:

• Common information about the algorithm can be in an intro section at the beginning, rather than in a duplicate doc entry for SequenceMatcher. The intro could also mention what a diff is
• There are also two doc entries for Differ
• Examples can be grouped together

Linked PRs

• gh-154661: Reorganize difflib documentation #154662
• [3.15] gh-154661: Reorganize difflib documentation (GH-154662) #154805
• [3.14] gh-154661: Reorganize difflib documentation (GH-154662) #154806
• [3.13] gh-154661: Reorganize difflib documentation (GH-154662) #154807