tools/docgen: Improve the code

[perillo]

• Remove dead contants.

• Remove the unused escapeHtml function.

• Remove the writeEscapedLines function, since line numbers where removed.

• Replace anytype with *Io.Writer when writing to a file, making the code more readable.

• Remove the escape parameter from the printShell function, since in docgen.zig it is always true.

• Reorganize imports.

[perillo]

I missed removing the unused escapeHtml function.
I missed removing the in function.

Probably I should remove the file.deprecatedReader() and use &in_file_reader.interface.readAlloc instead.

[perillo]

I missed the unused in function.

Additionally, in the main function, the use of deprecatedReader should be replace with the Io API

[perillo]

I missed the unused in function.

Additionally, in the main function, the use of deprecatedReader should be replaced with Io.Reader.

[perillo]

Closed due to #25036.

I will create a new PR for removing the dead code.