Minor fixes to outdated references in docs

There were some references to the GitHub issue tracker which is no
longer used, and a reference to a file which no longer existed. Also, a
broken link has been fixed.

Change-Id: Iad996dba88b8ce6fef95fdc57ba03696b17bef93
diff --git a/docs/contributing.md b/docs/contributing.md
index 0cdd4f2..9d452f0 100644
--- a/docs/contributing.md
+++ b/docs/contributing.md
@@ -57,7 +57,7 @@
 - [Build the extension](developers/build.md)
 - [Add a new feature](developers/add_feature.md)
 - [Submit changes for review](https://gerrit.avm99963.com/Documentation/intro-gerrit-walkthrough-github.html)
-- [How to test translations](https://developer.chrome.com/docs/extensions/reference/i18n/#how-to-set-your-browsers-locale)
+- [How to test translations](https://developer.chrome.com/docs/extensions/reference/i18n/#how-to-set-browsers-locale)
 *** promo
 TODO(avm99963): add a link to webext's `manifest.gjson` documentation when it is
@@ -66,20 +66,20 @@
 If you need any help, mentoring or anything else, send me an email at
 `me [at] avm99963.com` or create a thread in the
-[GitHub discussions tab](https://github.com/avm99963/infinitegforums/discussions).
+[Google group](https://groups.google.com/g/twpowertools-discuss).
 I will be more than happy to help you :)
 You can get a list of open issues which are recommended for first-time
 contributors here:
-- https://github.com/avm99963/infinitegforums/contribute
+- https://bugs.avm99963.com/p/twpowertools/issues/list?q=label%3AGoodFirstIssue
 These issues will let you get familiar with the codebase and are not too
 difficult to implement.
 You can also assign yourself any of the open issues which haven't been assigned:
-- https://github.com/avm99963/infinitegforums/issues?q=is%3Aissue+is%3Aopen+no%3Aassignee
+- https://bugs.avm99963.com/p/twpowertools/issues/list?q=status%3ANew%20OR%20status%3AAccepted
 For your information, the following doc contains a description of the release
 cycle for this extension, and how it is performed by its maintainers:
diff --git a/docs/developers/add_feature.md b/docs/developers/add_feature.md
index d8e93bb..16ebcee 100644
--- a/docs/developers/add_feature.md
+++ b/docs/developers/add_feature.md
@@ -14,10 +14,10 @@
     users have to explicitly enable the option after they receive the extension
     update. Otherwise, it might cause confusion, because users wouldn't know if
     the feature was added by the extension or Google.
-3. Now, modify the `//src/static/options/optionsPage.json5` file by adding an
-entry to the corresponding section. The _experimental_ property is optional and
-should only be used with features which are unreliable (or could be at some
-point in the future) due to their nature.
+3. Now, modify the `//src/options/optionsPage.json5` file by adding an entry to
+the corresponding section. The _experimental_ property is optional and should
+only be used with features which are unreliable (or could be at some point in
+the future) due to their nature.
 4. Finally, add the option string at `//src/static/_locales/en/manifest.json`,
 by adding the string under the `options_{{codename}}` key.