Improve site docs for `append` and add a separate `appendNulls` page

The current `append` site page exists but is very brief: https://kotlin.github.io/dataframe/append.html

Please improve the `append` documentation and add a separate documentation page for `appendNulls` instead of mixing both APIs on one page.

### Scope

- Expand the `append` page with:
- a short behavior description
- function signature
- input/output data tables, similar to other API pages such as `add`
- examples for appending one row and multiple rows
- a note that `append` returns a new `DataFrame` and does not mutate the original one
- a performance note: repeatedly appending rows in a loop is an antipattern; prefer building data first or appending/concatenating in batches

- Add a separate `appendNulls` page with:
- a short behavior description
- function signature
- input/output data tables
- examples for `appendNulls()` and `appendNulls(numberOfRows)`
- a note that `appendNulls(0)` returns the original dataframe
- a note that the operation is immutable and returns a new `DataFrame` when rows are added

### Acceptance criteria

- `append` page contains data tables and examples consistent with the style of pages like `add`.
- `appendNulls` has its own dedicated site page.
- Both pages explain immutability clearly.
- `append` page mentions the loop-append performance antipattern.
- Related operations/navigation are updated so both pages are discoverable.

Related to #1950 better to complete together