doc improvements (#8950)
* ignore direnv configs
https://direnv.net/ is a useful tool but its configs are developer-specific
* improve inferencing build docs
* Format build arguments and command line flags as code.
Without this, command line flags were being rendered with an
en dash `–` rather than two hyphens `--`, which meant copy-pasting
them would result in invalid command lines.
* Clarify how to avoid running tests.
* Minor formatting improvements.
* Update custom op docs
* Move contrib op example from PyTorch page to custom op page.
* On PyTorch page: document the use of pytorch_export_contrib_ops.
* On PyTorch page: Turn GitHub URLs into links.
Fixes #8818
