docs: Reduce the number of apparently broken links on github.

- Updated 260+ links from ".html" to ".md" to reduce the number of issues
reported about hyperlinks not working when viewing docs on Github.
- Removed temporary workaround that suppressed all warnings reported
by sphinx build for every link ending in ".html".

Details:
The recent upgrade to recommonmark==0.5.0 supports auto-converting
".md" links to ".html" so that the resulting HTML output is correct.

Notice that links pointing to a heading i.e. "../filename.html#heading",
were not updated because recommonmark does not auto-convert them.
These links do not generate build warnings and do not cause any issues.
However, there are about ~100 such links that might still get misreported
as broken links.  This will be a follow-up issue.

Background:
docs: pip upgrade recommonmark and CommonMark #13013
docs: Allow .md links between doc pages #11719

Fixes #11087.

docs/documentation/api.md

@@ -9,11 +9,11 @@ it and helping ensure it stays up to date as Zulip's API changes.
 Our API documentation is defined by a few sets of files:
 
 * Most data describing API endpoints and examples is stored in our
-  [OpenAPI configuration](../documentation/openapi.html) at
+  [OpenAPI configuration](../documentation/openapi.md) at
   `zerver/openapi/zulip.yaml`.
 * The top-level templates live under `templates/zerver/api/*`, and are
   written using the markdown framework that powers our [user
-  docs](../documentation/user.html), with some special extensions for
+  docs](../documentation/user.md), with some special extensions for
   rendering nice code blocks and example responses.
 * The text for the Python examples comes from a test suite for the
   Python API documentation (`zerver/openapi/python_examples.py`; run via
@@ -174,7 +174,7 @@ This section offers a step-by-step process for adding documentation
 for a new API endpoint.  It assumes you've read and understood the
 above.
 
-1. Start by adding [OpenAPI format](../documentation/openapi.html)
+1. Start by adding [OpenAPI format](../documentation/openapi.md)
    data to `zerver/openapi/zulip.yaml` for the endpoint.  If you
    copy-paste (which is helpful to get the indentation structure
    right), be sure to update all the content that you copied to

docs/documentation/overview.md

@@ -92,19 +92,19 @@ to do the things one does a lot in each type of documentation.
 
 To learn more about Zulip's general user documentation,
 [visit it on zulipchat.com](https://zulipchat.com/help/) or
-[read our guide on writing user documentation](user.html).
+[read our guide on writing user documentation](user.md).
 
 ### Integrations documentation
 
 To learn more about Zulip's integrations documentation,
 [visit it on zulipchat.com](https://zulipchat.com/integrations/) or
-[read our guide on writing user documentation](integrations.html).
+[read our guide on writing user documentation](integrations.md).
 
 ### API documentation
 
 To learn more about Zulip's API documentation,
 [visit it on zulipchat.com](https://zulipchat.com/api/) or
-[read our tutorial on writing user documentation](../documentation/api.html).
+[read our tutorial on writing user documentation](../documentation/api.md).
 
 ## Automated testing
 
@@ -114,7 +114,7 @@ recommend running locally when making significant edits:
 * `tools/lint` catches a number of common mistakes, and we highly
 recommend
 [using our linter pre-commit hook](../git/zulip-tools.html#set-up-git-repo-script).
-See the [main linter doc](../testing/linters.html) for more details.
+See the [main linter doc](../testing/linters.md) for more details.
 
 * The ReadTheDocs docs are built and the links tested by
 `tools/test-documentation`, which runs `build-docs` and then checks