Move skills to pipecat-ai/skills repo, add README instructions

Remove bundled Claude Code skills (changelog, cleanup, code-review,
docstring, pr-description, pr-submit) that now live in
https://github.com/pipecat-ai/skills. Add a section to the README
with installation instructions. The update-docs skill remains as
it is specific to this repository.

.claude/skills/pr-submit/SKILL.md

@@ -1,28 +0,0 @@
----
-name: pr-submit
-description: Create and submit a GitHub PR from the current branch
----
-
-Submit the current changes as a GitHub pull request.
-
-## Instructions
-
-1. Check the current state of the repository:
-   - Run `git status` to see staged, unstaged, and untracked changes
-   - Run `git diff` to see current changes
-   - Run `git log --oneline -10` to see recent commits
-
-2. If there are uncommitted changes relevant to the PR:
-   - Ask the user if they want a specific prefix for the branch name (e.g., `alice/`, `fix/`, `feat/`)
-   - Create a new branch based on the current branch
-   - Commit the changes using multiple commits if the changes are unrelated
-
-3. Push the branch and create the PR:
-   - Push with `-u` flag to set upstream tracking
-   - Create the PR using `gh pr create`
-
-4. After the PR is created:
-   - Run `/changelog <pr_number>` to generate changelog files, then commit and push them
-   - Run `/pr-description <pr_number>` to update the PR description
-
-5. Return the PR URL to the user.