DOC: Uniform block directive syntax.
block directive syntax technically does not have space before the
double-colon.

While this seem to be accepted by docutils – but not documented,
this can throw off other RST parser/syntax highlighter.
Sometime they can see that as comments, sometime they give garbage out.

Plus this is really uncommon with 3 occurrences in pandas codebase with
space and 4800 without:

    pandas[master]  $ rg '\.\. \w+::' | wc -l
    4853

Numpy and scipy used to have a few space in their block directive, but
current master/main branch of both should also be free of directive with
space before the double colon.

---

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew entry

Note that this issue can be seen in the syntax highlighting of the diff of this PR itself, where the deleted lines are gray (comment), and added ones are blue/purple (directives)


cc @MarcoGorelli  > could u add as a doc check rule as well (can be a follow up)

Yeah, I'm unsure how to currently check for that. I detected it automatically but the project that made me discover that is way too far from prime time to be added as a check. I could just grep for it and check the status code, but I'm afraid it would be a bit brittle. Could you add it as a pygrep check to the precommit config file?  Done.  lgtm. cc @MarcoGorelli  thanks @Carreau 