Fix broken links in documentation

There was a somewhat recent PR in the Guidelines that converted all link anchors to lowercase: https://github.com/isocpp/CppCoreGuidelines/pull/2304. This breaks many links in our documentation:

(cmd)carsonradtke@CARSON-MSFT:GSL $ git grep -n --full-name 'CppCoreGuidelines#[A-Z]'
docs/headers.md:5:See [GSL: Guidelines support library](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#S-gsl)
docs/headers.md:41:See [GSL.assert: Assertions](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-assertions)
docs/headers.md:89:See [SL.str.5: Use `std::byte` to refer to byte values that do not necessarily represent characters](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rstr-byte)
docs/headers.md:167:See [GSL.util: Utilities](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-utilities)
docs/headers.md:183:See [ES.46: Avoid lossy (narrowing, truncating) arithmetic conversions](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-narrowing) and [ES.49: If you must use a cast, use a named cast](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-casts-named)
docs/headers.md:189:See [GSL.view](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-views)
docs/headers.md:201:See [GSL.owner: Ownership pointers](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-ownership)
docs/headers.md:207:See [GSL.owner: Ownership pointers](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-ownership)
docs/headers.md:216:See Enforcement section of [C.31: All resources acquired by a class must be released by the class’s destructor](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rc-dtor-release).
docs/headers.md:225:See [F.23: Use a `not_null<T>` to indicate that “null” is not a valid value](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rf-nullptr)
docs/headers.md:281:Explicitly deleted operators. Pointers point to single objects ([I.13: Do not pass an array as a single pointer](http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Ri-array)), so don't allow these operators.
docs/headers.md:303:Array index operator is explicitly deleted. Pointers point to single objects ([I.13: Do not pass an array as a single pointer](http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Ri-array)), so don't allow treating them as an array.
docs/headers.md:378:Addition and subtraction are explicitly deleted. Pointers point to single objects ([I.13: Do not pass an array as a single pointer](http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Ri-array)), so don't allow these operators.
docs/headers.md:787:See [GSL.view](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-views) and [SL.str.3: Use zstring or czstring to refer to a C-style, zero-terminated, sequence of characters](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rstr-zstring).
docs/headers.md:794:See [GSL.util: Utilities](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#SS-utilities)
docs/headers.md:810:See [ES.46: Avoid lossy (narrowing, truncating) arithmetic conversions](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-narrowing) and [ES.49: If you must use a cast, use a named cast](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-casts-named)
docs/headers.md:821:See [E.19: Use a final_action object to express cleanup if no suitable resource handle is available](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Re-finally)
docs/headers.md:861:See [ES.42: Keep use of pointers simple and straightforward](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-ptr)

First and foremost, we should update these links so they resolve to the correct place. (Fixed by #1234)

Second, it would be nice if we had a script that could be run from a workflow that scans all links in the codebase and ensures they resolve and point to a valid anchor.

Review docs/headers.md and the lowercase-anchor change in PR #2304, noting that the broken-link updates were marked fixed by #1234. Then investigate the repository's workflow configuration and determine how a link-and-anchor scan could run there; done means the remaining documentation links are checked automatically and invalid targets are reported.