Fix /changelog/ and /contributing/ docs URLs (#1541)

* Fix `/changelog/` and `/contributing/` docs URLs

PR #1474 moved the docs to the `docs/` directory, and also changed
casing of some filenames.

- `docs/changelog.md` -> `docs/CHANGELOG.md`
- `docs/contributing.md` -> `docs/CONTRIBUTING.md`

This changed URL path parameters as well. URL paths to these pages must
now be uppercased when accessing the deployed docs (#1540).

For example, navigating to https://pipx.pypa.io/stable/CONTRIBUTING/
shows the correct page, but navigating to
https://pipx.pypa.io/stable/contributing/ displays a (poorly styled)
`404` page.

This commit will move the pages back to `docs/changelog.md` and
`docs/contributing.md` to restore the previous URLs to these pages.

Note that the casing changes to not affect the homepage because MkDocs
automatically handles `README.md` as `index.md`.
https://www.mkdocs.org/user-guide/writing-your-docs/#index-pages

* Categorize Towncrier changelog fragment as `doc`

#1541 (comment)

Author: br3ndonland

changelog.d/1540.doc.md

@@ -0,0 +1 @@
+Fix `/changelog/` and `/contributing/` docs URLs

docs/CHANGELOG.md docs/changelog.md

@@ -35,8 +35,8 @@ nav:
   - Comparison to Other Tools: "comparisons.md"
   - How pipx works: "how-pipx-works.md"
   - Programs to Try: "programs-to-try.md"
-  - Contributing: "CONTRIBUTING.md"
-  - Changelog: "CHANGELOG.md"
+  - Contributing: "contributing.md"
+  - Changelog: "changelog.md"
 
 markdown_extensions:
   - markdown_gfm_admonition  # GitHub's admonition (alert) syntax

docs/CONTRIBUTING.md docs/contributing.md

@@ -111,7 +111,7 @@ addopts = [ "-ra", "--strict-config", "--strict-markers" ]
 
 [tool.towncrier]
 directory = "changelog.d"
-filename = "docs/CHANGELOG.md"
+filename = "docs/changelog.md"
 start_string = "<!-- towncrier release notes start -->\n"
 underlines = [
   "",