Modify index preprocessor to also fix internal links pointing to

`README` files

Previously, the `index` preprocessor only changed the file names of
`README` to `index.md`, but did not modify internal links pointing to
said files to point to the renamed file instead. This resulted in
internal links to `README` files being broken and giving 404 errors.

This commit rectifies this by using the
`mdbook-markdown`/`pulldown-cmark` to find links within the book, and
the `url` crate to filter out internal links to `README` files, which
are then adjusted accordingly.

Fixes #984.

Author: CCJK123

Cargo.lock

@@ -68,6 +68,7 @@ topological-sort = "0.2.2"
 tower-http = "0.6.8"
 tracing = "0.1.44"
 tracing-subscriber = { version = "0.3.22", features = ["env-filter"] }
+url = "2.5.8"
 walkdir = "2.5.0"
 
 [package]

Cargo.toml

@@ -16,6 +16,7 @@ mdbook-markdown.workspace = true
 mdbook-preprocessor.workspace = true
 mdbook-renderer.workspace = true
 mdbook-summary.workspace = true
+pulldown-cmark-to-cmark.workspace = true
 regex.workspace = true
 serde.workspace = true
 serde_json.workspace = true
@@ -24,6 +25,7 @@ tempfile.workspace = true
 toml.workspace = true
 topological-sort.workspace = true
 tracing.workspace = true
+url.workspace = true
 
 [lints]
 workspace = true