linter: Add check for comma after "e.g." in md files.

Adds a linter rule for markdown files so that new cases of "e.g."
without a comma wont be introduced to user-facing documentation.

Fixes the remaining cases of "e.g." in markdown files that are not
followed by a comma.

CODE_OF_CONDUCT.md

@@ -66,7 +66,7 @@ organizers may take any action they deem appropriate, up to and including a
 temporary ban or permanent expulsion from the community without warning (and
 without refund in the case of a paid event).
 
-If someone outside the development community (e.g. a user of the Zulip
+If someone outside the development community (e.g., a user of the Zulip
 software) engages in unacceptable behavior that affects someone in the
 community, we still want to know. Even if we don't have direct control over
 the violator, the community organizers can still support the people

requirements/README.md

@@ -11,7 +11,7 @@ docs,pip
 Of the files, only dev and prod have been used in the install
 scripts directly. The rest are implicit dependencies.
 
-Steps to update a lock file, e.g. to update ipython from 5.3.0 to latest version:
+Steps to update a lock file, e.g., to update ipython from 5.3.0 to latest version:
 
 0. Remove entry for `ipython==5.3.0` in dev.txt.
 1. Run `./tools/update-locked-requirements`, which will generate new entries, pinned to the latest version.

static/images/logo/README.md

@@ -2,7 +2,7 @@ Generally, we prefer to use SVG assets when possible.
 
 However, many websites where you might want to use a Zulip logo do not
 support SVG files. If you need a Zulip logo asset in a different
-format (E.g. a 512px height PNG), you can generate that from one of
+format (e.g., a 512px height PNG), you can generate that from one of
 the `.svg` files in this directory.
 
 On Linux, you can generate a PNG of a given height using the following:

tools/droplets/README.md

@@ -66,7 +66,7 @@ listing all the droplets created during GCI.
 [Tags](https://www.digitalocean.com/community/tutorials/how-to-tag-digitalocean-droplets)
 may contain letters, numbers, colons, dashes, and underscores.
 
-You'll need to run this from the Zulip development environment (e.g. in
+You'll need to run this from the Zulip development environment (e.g., in
 Vagrant).
 
 The script will also stop if a droplet has already been created for the
@@ -130,7 +130,7 @@ so they are notified.
    - Check "Monitoring", "IPv6", and "User data"
    - Paste the contents of `tools/droplets/new-droplet-image` into the
      text box which says `Enter user data here...`
-   - Name it e.g. `base-ubuntu-22-04.zulipdev.org`
+   - Name it e.g., `base-ubuntu-22-04.zulipdev.org`
 1. Wait for the host to boot.
 1. Wait for the host to complete provisioning and shut down; this will take
    about 15 minutes.

tools/linter_lib/custom_check.py

@@ -879,6 +879,10 @@ markdown_rules = RuleList(
             "include_only": {"docs/"},
             "description": "Don't link directly to line numbers",
         },
+        {
+            "pattern": r"[eE]\.g\.[^,]",
+            "description": "Likely missing comma after 'e.g.'",
+        },
     ],
 )