docs: Reorganize documentation with new reference and explanation sections (#176)

Co-authored-by: Claude <noreply@anthropic.com>

docs/explanation/slug-transformation-rules.md

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+# Slug Transformation Rules
+
+This page explains the transformation algorithms used by the GitHub Slug Action, why they exist, and how they differ from each other.
+
+## What is a "slug"?
+
+A slug is a human-readable identifier derived from a string, safe for use in URLs, filenames, DNS labels, and other contexts where special characters are problematic. The term originates from publishing, where a "slug" is a short label used to identify a piece of content.
+
+In CI/CD workflows, branch names, tag names, and repository names often contain characters (`/`, `@`, spaces) that break when used in URLs, Docker tags, Kubernetes labels, or file paths. Slugifying these values makes them safe for such contexts.
+
+## SLUG transformation
+
+The `SLUG` transformation applies the following steps in order:
+
+1. **Lowercase** the entire string
+2. **Replace** any character that is not `0-9`, `a-z`, `.`, or `_` with `-`
+3. **Remove** leading `-` characters
+4. **Truncate** to a maximum length (default: 63 characters)
+5. **Remove** trailing `-` characters
+
+### SLUG examples
+
+| Input | Output |
+| ----- | ------ |
+| `refs/heads/feat/new_feature` | `feat-new-feature` |
+| `refs/tags/v1.0.0` | `v1.0.0` |
+| `refs/tags/product@1.0.0-rc.2` | `product-1.0.0-rc.2` |
+| `octocat/Hello-World` | `octocat-hello-world` |
+
+Note that `.` and `_` are preserved in SLUG output.
+
+## SLUG_URL transformation
+
+The `SLUG_URL` transformation is identical to `SLUG` except that `.` and `_` are also replaced with `-` in step 2. This makes the result safe for use as a URL path segment or subdomain label, where dots and underscores can cause issues.
+
+### SLUG_URL examples
+
+| Input | SLUG | SLUG_URL |
+| ----- | ---- | -------- |
+| `refs/tags/v1.0.0` | `v1.0.0` | `v1-0-0` |
+| `rlespinasse/Hello-World.go` | `rlespinasse-hello-world.go` | `rlespinasse-hello-world-go` |
+| `refs/heads/feat/new_feature` | `feat-new-feature` | `feat-new-feature` |
+
+### When to use which
+
+- Use **SLUG** for Docker tags, file paths, and general identifiers (dots and underscores are allowed)
+- Use **SLUG_URL** for subdomains, URL path segments, and any context where dots or underscores are problematic
+
+## Why 63 characters?
+
+The default maximum length of 63 comes from the [DNS label length limit](https://www.rfc-editor.org/rfc/rfc1035#section-2.3.4). This is also the maximum length for:
+
+- Kubernetes resource labels
+- Many cloud provider resource names
+- DNS subdomain components
+
+You can change this limit with the `slug-maxlength` input, or set it to `"nolimit"` to disable truncation entirely.
+
+## SHORT transformation
+
+The `SHORT` transformation truncates Git commit SHAs to a shorter, still-unique prefix. By default, Git determines the optimal length based on your repository size using [`git rev-parse --short`][git-revparse].
+
+| Input | Output |
+| ----- | ------ |
+| `ffac537e6cbbf934b08745a378932722df287a53` | `ffac537e` |
+
+The length varies by repository because larger repositories need longer prefixes to avoid collisions. You can set a fixed length with the `short-length` input (minimum: 4).
+
+## Case-sensitive variants (_CS)
+
+Every `SLUG` and `SLUG_URL` variable also has a `_CS` (case-sensitive) variant that skips the lowercase step. For example:
+
+| Input | SLUG | SLUG_CS |
+| ----- | ---- | ------- |
+| `refs/heads/New_Awesome_Product` | `new-awesome-product` | `New-Awesome-Product` |
+
+This is useful when the original casing carries meaning (e.g., product names).
+
+## PART extraction
+
+The `PART` transformation splits a composite value and extracts a portion. Currently used for `GITHUB_REPOSITORY`:
+
+| Input | OWNER_PART | NAME_PART |
+| ----- | ---------- | --------- |
+| `octocat/Hello-World` | `octocat` | `Hello-World` |
+
+PART variables preserve the original casing. They can be further transformed using SLUG or SLUG_URL suffixes (e.g., `GITHUB_REPOSITORY_OWNER_PART_SLUG`).
+
+[git-revparse]: https://git-scm.com/docs/git-rev-parse#Documentation/git-rev-parse.txt---shortlength