[chore][GraphQL] Clean-up unused commands

## Description

Remove commands to generate examples, markdown and schema from the
main binary as we do not use them:

- Instead of generating examples, we have hand-crafted examples in our
  docs. Removing this code also removes a test that forces
  regeneration of a markdown file from these docs (which we also were
  not using).
- We also never used the output from the `generate-schema`
  sub-command, because the schema was always available as a file, or
  via introspection commands from the running service.
- Logic for gathering examples to test has been moved into the test
  file, to avoid including test-only code in the main library.

## Test plan

CI

Cargo.toml

@@ -368,7 +368,6 @@ jsonrpsee = { git = "https://github.com/wlmyng/jsonrpsee.git", rev = "b1b3007847
 json_to_table = { git = "https://github.com/zhiburt/tabled/", rev = "e449317a1c02eb6b29e409ad6617e5d9eb7b3bd4" }
 leb128 = "0.2.5"
 lru = "0.10"
-markdown-gen = "1.2.1"
 match_opt = "0.1.2"
 miette = { version = "7", features = ["fancy"] }
 mime = "0.3"

crates/sui-graphql-rpc/Cargo.toml

@@ -32,7 +32,6 @@ lru.workspace = true
 move-binary-format.workspace = true
 move-disassembler.workspace = true
 move-ir-types.workspace = true
-markdown-gen.workspace = true
 mysten-metrics.workspace = true
 mysten-network.workspace = true
 move-core-types.workspace = true