fix(docs): handle fields examples with md_in_html

[agilgur5]

Fixes #12453

Motivation

• previously this used a hacky script, hack/parse_examples.go to make markdown work inside of the HTML details element

  • this hack is not run on RTD's build system, and so that meant the markdown ended up rendered as raw text

• instead of using hacky workarounds, use the md_in_html extension for mkdocs, which will render the markdown inside of the details elements correctly

  • this should also work on RTD seamlessly, as it's not a Go script exclusive to our Makefile

Modifications

• add a markdown attribute to the details element in hack/docgen.go per https://squidfunk.github.io/mkdocs-material/setup/extensions/python-markdown/#markdown-in-html

  • also remove new line that added too much space and is no longer necessary
    • might've been necessary with the hack as I don't remember it creating extra space before

• remove hack/parse_examples.go and all references to it

• plus make sure extensions are alphabetized

  • pymdownx.details should be before pymdownx.superfences

Verification

1. Ran make codegen (make docs/fields.md more specifically) to get the changes to docs/fields.md that are here in this diff
2. Then ran mkdocs build --strict to replicate the RTD build (i.e. not make docs)
3. Then open site/fields/index.html and verified the examples worked correctly as in below screenshot:

Before:

After:

Still works properly inside of GH Markdown as well -- see rendered MD on this PR

[jmeridth]

LGTM