rustdoc: add unstable latex math markdown feature

[notriddle]

Implement rust-lang/rfcs#3958

[rust-log-analyzer]

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

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[RUSTC-TIMING] unit_prefix test:false 0.058
error[E0532]: expected unit struct, unit variant or constant, found tuple variant `TagEnd::BlockQuote`
    --> src/tools/clippy/clippy_lints/src/doc/mod.rs:1150:29
     |
1150 |                         End(TagEnd::BlockQuote) => next_range.end,
     |                             ^^^^^^^^^^^^^^^^^^ help: use the tuple variant pattern syntax instead: `TagEnd::BlockQuote(_)`
     |
    ::: /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/pulldown-cmark-0.13.3/src/lib.rs:388:5
     |
 388 |     BlockQuote(Option<BlockQuoteKind>),
     |     ---------- `TagEnd::BlockQuote` defined here

[RUSTC-TIMING] bstr test:false 0.398
   Compiling pulldown-cmark v0.11.3
error[E0532]: expected unit struct, unit variant or constant, found tuple variant `TagEnd::BlockQuote`
    --> src/tools/clippy/clippy_lints/src/doc/mod.rs:1174:17
     |
1174 |             End(TagEnd::BlockQuote) => {
     |                 ^^^^^^^^^^^^^^^^^^ help: use the tuple variant pattern syntax instead: `TagEnd::BlockQuote(_)`
     |
    ::: /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/pulldown-cmark-0.13.3/src/lib.rs:388:5
     |
 388 |     BlockQuote(Option<BlockQuoteKind>),
     |     ---------- `TagEnd::BlockQuote` defined here

[RUSTC-TIMING] rustc_tools_util test:false 0.066
    Checking spanned v0.4.1
[RUSTC-TIMING] build_script_build test:false 0.094
    Checking indicatif v0.18.4
---
[RUSTC-TIMING] ui_test test:false 0.999
error[E0061]: this function takes 1 argument but 0 arguments were supplied
   --> src/tools/clippy/clippy_lints/src/doc/doc_paragraphs_missing_punctuation.rs:59:37
    |
 59 |         Parser::new_ext(doc_string, main_body_opts() - Options::ENABLE_SMART_PUNCTUATION).into_offset_iter()
    |                                     ^^^^^^^^^^^^^^-- argument #1 of type `std::option::Option<&DocAttributeSyntax>` is missing
    |
note: function defined here
   --> compiler/rustc_resolve/src/rustdoc.rs:252:8
    |
252 | pub fn main_body_opts(doc_syntax: Option<&DocAttributeSyntax>) -> Options {
    |        ^^^^^^^^^^^^^^
help: provide the argument
    |
 59 |         Parser::new_ext(doc_string, main_body_opts(/* std::option::Option<&DocAttributeSyntax> */) - Options::ENABLE_SMART_PUNCTUATION).into_offset_iter()
    |                                                    ++++++++++++++++++++++++++++++++++++++++++++++

error[E0061]: this function takes 1 argument but 0 arguments were supplied
   --> src/tools/clippy/clippy_lints/src/doc/mod.rs:915:17
    |
915 |                 main_body_opts() - Options::ENABLE_SMART_PUNCTUATION,
    |                 ^^^^^^^^^^^^^^-- argument #1 of type `std::option::Option<&DocAttributeSyntax>` is missing
    |
note: function defined here
   --> compiler/rustc_resolve/src/rustdoc.rs:252:8
    |
252 | pub fn main_body_opts(doc_syntax: Option<&DocAttributeSyntax>) -> Options {
    |        ^^^^^^^^^^^^^^
help: provide the argument
    |
915 |                 main_body_opts(/* std::option::Option<&DocAttributeSyntax> */) - Options::ENABLE_SMART_PUNCTUATION,
    |                                ++++++++++++++++++++++++++++++++++++++++++++++

error[E0061]: this function takes 1 argument but 0 arguments were supplied
   --> src/tools/clippy/clippy_lints/src/doc/mod.rs:947:16
    |
947 |     let opts = main_body_opts() - Options::ENABLE_SMART_PUNCTUATION;
    |                ^^^^^^^^^^^^^^-- argument #1 of type `std::option::Option<&DocAttributeSyntax>` is missing
    |
note: function defined here
   --> compiler/rustc_resolve/src/rustdoc.rs:252:8
    |
252 | pub fn main_body_opts(doc_syntax: Option<&DocAttributeSyntax>) -> Options {
    |        ^^^^^^^^^^^^^^
help: provide the argument
    |
947 |     let opts = main_body_opts(/* std::option::Option<&DocAttributeSyntax> */) - Options::ENABLE_SMART_PUNCTUATION;
    |                               ++++++++++++++++++++++++++++++++++++++++++++++

[RUSTC-TIMING] askama test:false 0.286
Some errors have detailed explanations: E0061, E0532.
For more information about an error, try `rustc --explain E0061`.