docs: fix typos in comments, docstrings and CLI help text

[Avicennasis]

Summary

Fixes six typos across six files. No functional changes - comments, docstrings, documentation and user-facing help/error strings only.

Reported findings

• migration_lint/analyzer/base.py: docstring Perform SQL migartion linting. -> Perform SQL migration linting.
• migration_lint/main.py: --gitlab-instance help text GitLab instance instance (protocol://host:port) -> GitLab instance (protocol://host:port) (duplicated word)
• migration_lint/django/management/commands/migration_lint.py: the same duplicated word in the Django management command's --gitlab-instance help text
• migration_lint/extractor/base.py: NotImplementedError message doesn't provie name -> doesn't provide name

Additionally noticed

While verifying the files above I found two more typos in the same repository and fixed them in this branch:

• migration_lint/extractor/model.py: docstring An object desribing a single changed file. -> An object describing a single changed file.
• docs/index.md: added or modified and not yet commited. -> ... and not yet committed.

Notes

• The extractor/base.py change touches an exception message rather than a comment. I grepped tests/ for both provie and provide name and found no assertion that matches on that string, so no test changes were needed.
• The two --gitlab-instance help strings are user-visible CLI output; the flag name itself is unchanged.
• No identifiers were renamed and no generated or vendored files were touched.