DOC: PDEP6 page format update
- [x] closes #53414
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

No preview found for PR #53442. Did the docs build complete? No preview found for PR #53442. Did the docs build complete? @MarcoGorelli, here is the link to preview the change:

https://pandas.pydata.org/preview/53442/pdeps/0006-ban-upcasting.html

[77d7834](https://github.com/pandas-dev/pandas/pull/53442/commits/77d7834f9fe02caa52995b405446ccae348f0f4e) updates the list format for the occurrence which was mentioned in the issue.
[ddf551a](https://github.com/pandas-dev/pandas/pull/53442/commits/ddf551a6fcdad4ae5205d57f975a6a8fcf9670b2) updates the entire document format where bulleted lists are suggested.
[b976bc5](https://github.com/pandas-dev/pandas/pull/53442/commits/b976bc5501f5cb21833a843b570d3e6fd96ce44c) fixes unintended deletion.
[478e2e9](https://github.com/pandas-dev/pandas/pull/53442/commits/478e2e91e5f4a133ebcf4508ace2b4ef965a4fda) includes a `'` for (`Series`' dtype) in line 12 reflecting grammar rules expressed in line 92.
[9f345e8](https://github.com/pandas-dev/pandas/pull/53442/commits/9f345e8d2f23aff2ba4c70538590dcf4a627b9ed) includes formatting for numbered list, and punctuation for list objects where appropriate.
[fa8cf3a](https://github.com/pandas-dev/pandas/pull/53442/commits/fa8cf3a34fd1b273b2cf2639eefdf6158fc48bfb) includes proper formatting for nested list (2) in detailed description section of PDEP6.
[7722182](https://github.com/pandas-dev/pandas/pull/53442/commits/772218239ffd081d2e6a421838021edc76722763) includes final formatting changes utilizing local build, resulting in accurate rendering.