docs: Convert .html#fragment links to .md#fragment.

This uses the myst_heading_anchors option to automatically generate
header anchors and make Sphinx aware of them.  See
https://myst-parser.readthedocs.io/en/latest/syntax/optional.html#auto-generated-header-anchors.

Note: to be compatible with GitHub, MyST-Parser uses a slightly
different convention for .md fragment links than .html fragment links
when punctuation is involved.  This does not affect the generated
fragment links in the HTML output.

Fixes #13264.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

docs/translating/internationalization.md

@@ -26,7 +26,7 @@ principles are important in how we think about internationalization:
   element needs to be built in a way that supports i18n.
 - This is more about string consistency in general, but we have a
   "Sentence case" [capitalization
-  policy](translating.html#capitalization) that we enforce using linters
+  policy](translating.md#capitalization) that we enforce using linters
   that check all strings tagged for translation in Zulip.
 
 This article aims to provide a brief introduction. We recommend the
@@ -108,7 +108,7 @@ The end-to-end tooling process for translations in Zulip is as follows.
    Transifex API tool, `tx pull`, internally).
 
 If you're interested, you may also want to check out the [translators'
-workflow](translating.html#translators-workflow), just so you have a
+workflow](translating.md#translators-workflow), just so you have a
 sense of how everything fits together.
 
 ## Translation resource files
@@ -339,4 +339,4 @@ organizations from the command line.
 [helpers]: https://handlebarsjs.com/guide/block-helpers.html
 [transifex]: https://transifex.com
 [transifexrc]: https://docs.transifex.com/client/client-configuration#transifexrc
-[html-templates]: ../subsystems/html-css.html#html-templates
+[html-templates]: ../subsystems/html-css.md#html-templates